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NAME

       dets - A disk-based term storage.

DESCRIPTION

       This  module  provides  a  term  storage  on file. The stored terms, in this module called
       objects, are tuples such that one element is defined to be the key.  A  Dets  table  is  a
       collection of objects with the key at the same position stored on a file.

       This  module  is used by the Mnesia application, and is provided "as is" for users who are
       interested in efficient storage of Erlang terms on disk only. Many applications only  need
       to  store  some  terms in a file. Mnesia adds transactions, queries, and distribution. The
       size of Dets files cannot exceed 2 GB. If larger tables are needed, table fragmentation in
       Mnesia can be used.

       Three types of Dets tables exist:

         * set. A table of this type has at most one object with a given key. If an object with a
           key already present in the table is inserted, the existing object  is  overwritten  by
           the new object.

         * bag. A table of this type has zero or more different objects with a given key.

         * duplicate_bag.  A table of this type has zero or more possibly matching objects with a
           given key.

       Dets tables must be opened before they can be updated or read, and when finished they must
       be  properly  closed.  If  a  table is not properly closed, Dets automatically repairs the
       table. This can take a substantial time if the table is large. A Dets table is closed when
       the  process  which opened the table terminates. If many Erlang processes (users) open the
       same Dets table, they share the table. The table is properly closed when  all  users  have
       either  terminated  or closed the table. Dets tables are not properly closed if the Erlang
       runtime system terminates abnormally.

   Note:
       A ^C command abnormally terminates an Erlang runtime system in a Unix environment  with  a
       break-handler.

       As all operations performed by Dets are disk operations, it is important to realize that a
       single look-up operation involves a series of disk seek  and  read  operations.  The  Dets
       functions  are  therefore much slower than the corresponding ets(3erl) functions, although
       Dets exports a similar interface.

       Dets organizes data as a linear hash list and the hash list grows gracefully as more  data
       is  inserted into the table. Space management on the file is performed by what is called a
       buddy system. The current implementation keeps the  entire  buddy  system  in  RAM,  which
       implies  that  if the table gets heavily fragmented, quite some memory can be used up. The
       only way to defragment a table is to close it and then open it again  with  option  repair
       set to force.

       Notice  that  type  ordered_set in Ets is not yet provided by Dets, neither is the limited
       support for concurrent updates that makes a sequence of first and next calls safe  to  use
       on  fixed  ETS  tables. Both these features may be provided by Dets in a future release of
       Erlang/OTP. Until then, the  Mnesia  application  (or  some  user-implemented  method  for
       locking)  must be used to implement safe concurrency. Currently, no Erlang/OTP library has
       support for ordered disk-based term storage.

       All Dets functions return {error, Reason} if an  error  occurs  (first/1  and  next/2  are
       exceptions,  they  exit  the  process with the error tuple). If badly formed arguments are
       specified, all functions exit the process with a badarg message.

DATA TYPES

       access() = read | read_write

       auto_save() = infinity | integer() >= 0

       bindings_cont()

              Opaque continuation used by match/1 and match/3.

       cont()

              Opaque continuation used by bchunk/2.

       keypos() = integer() >= 1

       match_spec() = ets:match_spec()

              Match specifications, see section  Match Specification in  Erlang  in  ERTS  User's
              Guide and the ms_transform(3erl) module.

       no_slots() = default | integer() >= 0

       object() = tuple()

       object_cont()

              Opaque continuation used by match_object/1 and match_object/3.

       pattern() = atom() | tuple()

              For a description of patterns, see ets:match/2.

       select_cont()

              Opaque continuation used by select/1 and select/3.

       tab_name() = term()

       type() = bag | duplicate_bag | set

EXPORTS

       all() -> [tab_name()]

              Returns a list of the names of all open tables on this node.

       bchunk(Name, Continuation) ->
                 {Continuation2, Data} |
                 '$end_of_table' |
                 {error, Reason}

              Types:

                 Name = tab_name()
                 Continuation = start | cont()
                 Continuation2 = cont()
                 Data = binary() | tuple()
                 Reason = term()

              Returns  a  list  of  objects  stored  in  a table. The exact representation of the
              returned objects is not public. The lists of data can be used  for  initializing  a
              table  by  specifying  value  bchunk  to option format of function init_table/3 The
              Mnesia application uses this function for copying open tables.

              Unless the table is protected using safe_fixtable/2, calls to bchunk/2 do  possibly
              not work as expected if concurrent updates are made to the table.

              The first time bchunk/2 is called, an initial continuation, the atom start, must be
              provided.

              bchunk/2 returns a tuple {Continuation2, Data}, where Data is a  list  of  objects.
              Continuation2  is another continuation that is to be passed on to a subsequent call
              to bchunk/2. With a  series  of  calls  to  bchunk/2,  all  table  objects  can  be
              extracted.

              bchunk/2  returns '$end_of_table' when all objects are returned, or {error, Reason}
              if an error occurs.

       close(Name) -> ok | {error, Reason}

              Types:

                 Name = tab_name()
                 Reason = term()

              Closes a table. Only processes that have opened a table are allowed to close it.

              All open tables must be closed before the system is stopped. If an attempt is  made
              to open a table that is not properly closed, Dets automatically tries to repair it.

       delete(Name, Key) -> ok | {error, Reason}

              Types:

                 Name = tab_name()
                 Key = Reason = term()

              Deletes all objects with key Key from table Name.

       delete_all_objects(Name) -> ok | {error, Reason}

              Types:

                 Name = tab_name()
                 Reason = term()

              Deletes  all objects from a table in almost constant time. However, if the table if
              fixed, delete_all_objects(T) is equivalent to match_delete(T, '_').

       delete_object(Name, Object) -> ok | {error, Reason}

              Types:

                 Name = tab_name()
                 Object = object()
                 Reason = term()

              Deletes all instances of a specified object from a table. If a table is of type bag
              or duplicate_bag, this function can be used to delete only some of the objects with
              a specified key.

       first(Name) -> Key | '$end_of_table'

              Types:

                 Name = tab_name()
                 Key = term()

              Returns the first key stored in table Name according to the internal order  of  the
              table, or '$end_of_table' if the table is empty.

              Unless  the table is protected using safe_fixtable/2, subsequent calls to next/2 do
              possibly not work as expected if concurrent updates are made to the table.

              If an error occurs, the process is exited with an error tuple {error, Reason}.  The
              error tuple is not returned, as it cannot be distinguished from a key.

              There  are  two  reasons  why  first/1  and next/2 are not to be used: they are not
              efficient, and they prevent the use of key '$end_of_table', as this atom is used to
              indicate  the end of the table. If possible, use functions match, match_object, and
              select for traversing tables.

       foldl(Function, Acc0, Name) -> Acc | {error, Reason}

       foldr(Function, Acc0, Name) -> Acc | {error, Reason}

              Types:

                 Name = tab_name()
                 Function = fun((Object :: object(), AccIn) -> AccOut)
                 Acc0 = Acc = AccIn = AccOut = Reason = term()

              Calls Function on successive elements of table Name together with an extra argument
              AccIn.  The table elements are traversed in unspecified order. Function must return
              a new accumulator that is passed to the next call. Acc0 is returned if the table is
              empty.

       from_ets(Name, EtsTab) -> ok | {error, Reason}

              Types:

                 Name = tab_name()
                 EtsTab = ets:tab()
                 Reason = term()

              Deletes all objects of table Name and then inserts all the objects of the ETS table
              EtsTab. The objects are inserted in unspecified order.  As  ets:safe_fixtable/2  is
              called, the ETS table must be public or owned by the calling process.

       info(Name) -> InfoList | undefined

              Types:

                 Name = tab_name()
                 InfoList = [InfoTuple]
                 InfoTuple =
                     {file_size, integer() >= 0} |
                     {filename, file:name()} |
                     {keypos, keypos()} |
                     {size, integer() >= 0} |
                     {type, type()}

              Returns information about table Name as a list of tuples:

                * {file_size, integer() >= 0}} - The file size, in bytes.

                * {filename, file:name()} - The name of the file where objects are stored.

                * {keypos, keypos()} - The key position.

                * {size, integer() >= 0} - The number of objects stored in the table.

                * {type, type()} - The table type.

       info(Name, Item) -> Value | undefined

              Types:

                 Name = tab_name()
                 Item =
                     access | auto_save | bchunk_format | hash | file_size |
                     filename | keypos | memory | no_keys | no_objects | no_slots |
                     owner | ram_file | safe_fixed | safe_fixed_monotonic_time |
                     size | type
                 Value = term()

              Returns  the  information  associated  with Item for table Name. In addition to the
              {Item, Value} pairs defined for info/1, the following items are allowed:

                * {access, access()} - The access mode.

                * {auto_save, auto_save()} - The autosave interval.

                * {bchunk_format, binary()} - An opaque  binary  describing  the  format  of  the
                  objects   returned  by  bchunk/2.  The  binary  can  be  used  as  argument  to
                  is_compatible_chunk_format/2.

                * {hash, Hash} - Describes which BIF is used to calculate the hash values of  the
                  objects stored in the Dets table. Possible values of Hash:

                  * phash - Implies that the erlang:phash/2 BIF is used.

                  * phash2 - Implies that the erlang:phash2/1 BIF is used.

                * {memory,  integer()  >=  0}  -  The  file  size,  in  bytes.  The same value is
                  associated with item file_size.

                * {no_keys, integer >= 0()} - The number of different keys stored in the table.

                * {no_objects, integer >= 0()} - The number of objects stored in the table.

                * {no_slots, {Min, Used, Max}} - The number of slots of the  table.  Min  is  the
                  minimum number of slots, Used is the number of currently used slots, and Max is
                  the maximum number of slots.

                * {owner, pid()} - The pid of the process  that  handles  requests  to  the  Dets
                  table.

                * {ram_file, boolean()} - Whether the table is kept in RAM.

                * {safe_fixed_monotonic_time,  SafeFixed} - If the table is fixed, SafeFixed is a
                  tuple {FixedAtTime, [{Pid,RefCount}]}. FixedAtTime is the time when  the  table
                  was  first  fixed,  and  Pid  is  the  pid  of the process that fixes the table
                  RefCount times. There can be any number of processes in the list. If the  table
                  is not fixed, SafeFixed is the atom false.

                  FixedAtTime  corresponds  to  the result returned by erlang:monotonic_time/0 at
                  the time of fixation. The use of safe_fixed_monotonic_time is  time warp safe.

                * {safe_fixed, SafeFixed} - The same  as  {safe_fixed_monotonic_time,  SafeFixed}
                  except the format and value of FixedAtTime.

                  FixedAtTime  corresponds  to  the  result returned by erlang:timestamp/0 at the
                  time of fixation. Notice that when the system uses single or  multi  time  warp
                  modes,  this can produce strange results. This is because the use of safe_fixed
                  is not  time warp safe. Time warp safe code must use  safe_fixed_monotonic_time
                  instead.

       init_table(Name, InitFun) -> ok | {error, Reason}

       init_table(Name, InitFun, Options) -> ok | {error, Reason}

              Types:

                 Name = tab_name()
                 InitFun = fun((Arg) -> Res)
                 Arg = read | close
                 Res =
                     end_of_input |
                     {[object()], InitFun} |
                     {Data, InitFun} |
                     term()
                 Options = Option | [Option]
                 Option = {min_no_slots, no_slots()} | {format, term | bchunk}
                 Reason = term()
                 Data = binary() | tuple()

              Replaces  the  existing  objects  of table Name with objects created by calling the
              input function InitFun, see below. The reason for using this function  rather  than
              calling  insert/2 is that of efficiency. Notice that the input functions are called
              by the process that handles requests to the Dets table, not by the calling process.

              When called with argument read, function InitFun is assumed to return  end_of_input
              when  there is no more input, or {Objects, Fun}, where Objects is a list of objects
              and Fun is a new input function. Any other value Value  is  returned  as  an  error
              {error,  {init_fun,  Value}}. Each input function is called exactly once, and if an
              error occurs, the last function is called with argument close, the reply  of  which
              is ignored.

              If  the  table type is set and more than one object exists with a given key, one of
              the objects is chosen. This is not necessarily the last object with the  given  key
              in  the  sequence of objects returned by the input functions. Avoid duplicate keys,
              otherwise the file becomes unnecessarily fragmented. This holds also for duplicated
              objects stored in tables of type bag.

              It is important that the table has a sufficient number of slots for the objects. If
              not, the hash list starts to grow when init_table/2  returns,  which  significantly
              slows down access to the table for a period of time. The minimum number of slots is
              set by the  open_file/2  option  min_no_slots  and  returned  by  the  info/2  item
              no_slots. See also option min_no_slots below.

              Argument  Options  is  a  list of {Key, Val} tuples, where the following values are
              allowed:

                * {min_no_slots, no_slots()} - Specifies the estimated number of  different  keys
                  to  be  stored  in  the  table.  The  open_file/2  option with the same name is
                  ignored, unless the table is created, in which case performance can be enhanced
                  by supplying an estimate when initializing the table.

                * {format,  Format}  -  Specifies  the format of the objects returned by function
                  InitFun. If Format is term (the default), InitFun is assumed to return  a  list
                  of  tuples.  If Format is bchunk, InitFun is assumed to return Data as returned
                  by bchunk/2. This option overrides option min_no_slots.

       insert(Name, Objects) -> ok | {error, Reason}

              Types:

                 Name = tab_name()
                 Objects = object() | [object()]
                 Reason = term()

              Inserts one or more objects into the table Name. If there already exists an  object
              with a key matching the key of some of the given objects and the table type is set,
              the old object will be replaced.

       insert_new(Name, Objects) -> boolean() | {error, Reason}

              Types:

                 Name = tab_name()
                 Objects = object() | [object()]
                 Reason = term()

              Inserts one or more objects into table Name. If there already  exists  some  object
              with  a  key  matching  the  key  of any of the specified objects, the table is not
              updated and false  is  returned.  Otherwise  the  objects  are  inserted  and  true
              returned.

       is_compatible_bchunk_format(Name, BchunkFormat) -> boolean()

              Types:

                 Name = tab_name()
                 BchunkFormat = binary()

              Returns  true  if it would be possible to initialize table Name, using init_table/3
              with option {format, bchunk}, with objects read with bchunk/2 from  some  table  T,
              such that calling info(T, bchunk_format) returns BchunkFormat.

       is_dets_file(Filename) -> boolean() | {error, Reason}

              Types:

                 Filename = file:name()
                 Reason = term()

              Returns true if file Filename is a Dets table, otherwise false.

       lookup(Name, Key) -> Objects | {error, Reason}

              Types:

                 Name = tab_name()
                 Key = term()
                 Objects = [object()]
                 Reason = term()

              Returns a list of all objects with key Key stored in table Name, for example:

              2> dets:open_file(abc, [{type, bag}]).
              {ok,abc}
              3> dets:insert(abc, {1,2,3}).
              ok
              4> dets:insert(abc, {1,3,4}).
              ok
              5> dets:lookup(abc, 1).
              [{1,2,3},{1,3,4}]

              If the table type is set, the function returns either the empty list or a list with
              one object, as there cannot be more than one object with a given key. If the  table
              type is bag or duplicate_bag, the function returns a list of arbitrary length.

              Notice  that the order of objects returned is unspecified. In particular, the order
              in which objects were inserted is not reflected.

       match(Continuation) ->
                {[Match], Continuation2} |
                '$end_of_table' |
                {error, Reason}

              Types:

                 Continuation = Continuation2 = bindings_cont()
                 Match = [term()]
                 Reason = term()

              Matches some objects stored in a table and returns a non-empty list of the bindings
              matching a specified pattern in some unspecified order. The table, the pattern, and
              the number of objects that are matched are all defined by Continuation,  which  has
              been returned by a previous call to match/1 or match/3.

              When all table objects are matched, '$end_of_table' is returned.

       match(Name, Pattern) -> [Match] | {error, Reason}

              Types:

                 Name = tab_name()
                 Pattern = pattern()
                 Match = [term()]
                 Reason = term()

              Returns  for  each  object of table Name that matches Pattern a list of bindings in
              some unspecified order. For a description of  patterns,  see  ets:match/2.  If  the
              keypos'th  element  of  Pattern  is  unbound, all table objects are matched. If the
              keypos'th element is bound, only the objects with the correct key are matched.

       match(Name, Pattern, N) ->
                {[Match], Continuation} |
                '$end_of_table' |
                {error, Reason}

              Types:

                 Name = tab_name()
                 Pattern = pattern()
                 N = default | integer() >= 0
                 Continuation = bindings_cont()
                 Match = [term()]
                 Reason = term()

              Matches some or all objects of table Name and  returns  a  non-empty  list  of  the
              bindings  that  match  Pattern  in  some  unspecified  order.  For a description of
              patterns, see ets:match/2.

              A tuple of the bindings and a continuation is returned, unless the table is  empty,
              in  which  case  '$end_of_table'  is  returned. The continuation is to be used when
              matching further objects by calling match/1.

              If the keypos'th element of Pattern is bound, all table objects are matched. If the
              keypos'th  element  is unbound, all table objects are matched, N objects at a time,
              until at least one object matches or the end of the table is reached. The  default,
              indicated  by  giving  N  the  value  default, is to let the number of objects vary
              depending on the sizes of the objects. All objects with the  same  key  are  always
              matched  at  the same time, which implies that more than N objects can sometimes be
              matched.

              The table is always to be protected using safe_fixtable/2 before  calling  match/3,
              otherwise errors can occur when calling match/1.

       match_delete(Name, Pattern) -> ok | {error, Reason}

              Types:

                 Name = tab_name()
                 Pattern = pattern()
                 Reason = term()

              Deletes  all  objects  that  match  Pattern  from  table Name. For a description of
              patterns, see ets:match/2.

              If the keypos'th element of Pattern is bound, only the objects with the correct key
              are matched.

       match_object(Continuation) ->
                       {Objects, Continuation2} |
                       '$end_of_table' |
                       {error, Reason}

              Types:

                 Continuation = Continuation2 = object_cont()
                 Objects = [object()]
                 Reason = term()

              Returns  a  non-empty  list  of  some  objects stored in a table that match a given
              pattern in some unspecified order. The  table,  the  pattern,  and  the  number  of
              objects  that  are matched are all defined by Continuation, which has been returned
              by a previous call to match_object/1 or match_object/3.

              When all table objects are matched, '$end_of_table' is returned.

       match_object(Name, Pattern) -> Objects | {error, Reason}

              Types:

                 Name = tab_name()
                 Pattern = pattern()
                 Objects = [object()]
                 Reason = term()

              Returns a list of all objects of table Name that match Pattern in some  unspecified
              order. For a description of patterns, see ets:match/2.

              If  the  keypos'th element of Pattern is unbound, all table objects are matched. If
              the keypos'th element of Pattern is bound, only the objects with  the  correct  key
              are matched.

              Using the match_object functions for traversing all table objects is more efficient
              than calling first/1 and next/2 or slot/2.

       match_object(Name, Pattern, N) ->
                       {Objects, Continuation} |
                       '$end_of_table' |
                       {error, Reason}

              Types:

                 Name = tab_name()
                 Pattern = pattern()
                 N = default | integer() >= 0
                 Continuation = object_cont()
                 Objects = [object()]
                 Reason = term()

              Matches some or all objects stored in table Name and returns a  non-empty  list  of
              the  objects  that  match  Pattern  in some unspecified order. For a description of
              patterns, see ets:match/2.

              A list of objects and a continuation is returned, unless the  table  is  empty,  in
              which  case  '$end_of_table'  is  returned.  The  continuation  is  to be used when
              matching further objects by calling match_object/1.

              If the keypos'th element of Pattern is bound, all table objects are matched. If the
              keypos'th  element  is unbound, all table objects are matched, N objects at a time,
              until at least one object matches or the end of the table is reached. The  default,
              indicated  by  giving  N  the  value  default, is to let the number of objects vary
              depending on the sizes of the objects. All matching objects with the same  key  are
              always  returned  in  the  same  reply,  which implies that more than N objects can
              sometimes be returned.

              The  table  is  always  to  be  protected  using  safe_fixtable/2  before   calling
              match_object/3, otherwise errors can occur when calling match_object/1.

       member(Name, Key) -> boolean() | {error, Reason}

              Types:

                 Name = tab_name()
                 Key = Reason = term()

              Works  like  lookup/2, but does not return the objects. Returns true if one or more
              table elements has key Key, otherwise false.

       next(Name, Key1) -> Key2 | '$end_of_table'

              Types:

                 Name = tab_name()
                 Key1 = Key2 = term()

              Returns either the key following Key1 in table Name according to the internal order
              of the table, or '$end_of_table' if there is no next key.

              If an error occurs, the process is exited with an error tuple {error, Reason}.

              To find the first key in the table, use first/1.

       open_file(Filename) -> {ok, Reference} | {error, Reason}

              Types:

                 Filename = file:name()
                 Reference = reference()
                 Reason = term()

              Opens  an  existing table. If the table is not properly closed, it is repaired. The
              returned reference is to be used as the table name. This function  is  most  useful
              for debugging purposes.

       open_file(Name, Args) -> {ok, Name} | {error, Reason}

              Types:

                 Name = tab_name()
                 Args = [OpenArg]
                 OpenArg =
                     {access, access()} |
                     {auto_save, auto_save()} |
                     {estimated_no_objects, integer() >= 0} |
                     {file, file:name()} |
                     {max_no_slots, no_slots()} |
                     {min_no_slots, no_slots()} |
                     {keypos, keypos()} |
                     {ram_file, boolean()} |
                     {repair, boolean() | force} |
                     {type, type()}
                 Reason = term()

              Opens a table. An empty Dets table is created if no file exists.

              The  atom Name is the table name. The table name must be provided in all subsequent
              operations on the table. The name can be used by other processes as well, and  many
              processes can share one table.

              If  two  processes  open  the same table by giving the same name and arguments, the
              table has two users. If one user closes the table, it remains open until the second
              user closes it.

              Argument  Args  is  a  list  of  {Key,  Val} tuples, where the following values are
              allowed:

                * {access, access()} - Existing tables can be opened in read-only mode.  A  table
                  that  is  opened  in  read-only  mode  is  not  subjected to the automatic file
                  reparation algorithm  if  it  is  later  opened  after  a  crash.  Defaults  to
                  read_write.

                * {auto_save, auto_save()} - The autosave interval. If the interval is an integer
                  Time, the table is flushed to  disk  whenever  it  is  not  accessed  for  Time
                  milliseconds.  A  table  that  has  been  flushed  requires  no reparation when
                  reopened after an uncontrolled emulator halt.  If  the  interval  is  the  atom
                  infinity, autosave is disabled. Defaults to 180000 (3 minutes).

                * {estimated_no_objects, no_slots()} - Equivalent to option min_no_slots.

                * {file,  file:name()} - The name of the file to be opened. Defaults to the table
                  name.

                * {max_no_slots, no_slots()} - The maximum number of slots to be  used.  Defaults
                  to  32  M,  which is the maximal value. Notice that a higher value can increase
                  the table fragmentation, and a smaller value can decrease the fragmentation, at
                  the expense of execution time.

                * {min_no_slots,  no_slots()} - Application performance can be enhanced with this
                  flag by specifying,  when  the  table  is  created,  the  estimated  number  of
                  different keys to be stored in the table. Defaults to 256, which is the minimum
                  value.

                * {keypos, keypos()} - The position of the element of each object to be  used  as
                  key.  Defaults  to  1. The ability to explicitly state the key position is most
                  convenient when we want to store Erlang records in which the first position  of
                  the record is the name of the record type.

                * {ram_file,  boolean()}  -  Whether  the table is to be kept in RAM. Keeping the
                  table in RAM can sound like an anomaly, but  can  enhance  the  performance  of
                  applications  that  open  a  table, insert a set of objects, and then close the
                  table. When the table is closed, its contents are written  to  the  disk  file.
                  Defaults to false.

                * {repair,  Value}  - Value can be either a boolean() or the atom force. The flag
                  specifies if the Dets  server  is  to  invoke  the  automatic  file  reparation
                  algorithm.  Defaults  to  true.  If  false  is specified, no attempt is made to
                  repair the file, and {error, {needs_repair, FileName}} is returned if the table
                  must be repaired.

                  Value  force  means  that  a  reparation  is made even if the table is properly
                  closed. This is a seldom needed option.

                  Option repair is ignored if the table is already open.

                * {type, type()} - The table type. Defaults to set.

       pid2name(Pid) -> {ok, Name} | undefined

              Types:

                 Pid = pid()
                 Name = tab_name()

              Returns the table name given the pid of a process that handles requests to a table,
              or undefined if there is no such table.

              This function is meant to be used for debugging only.

       repair_continuation(Continuation, MatchSpec) -> Continuation2

              Types:

                 Continuation = Continuation2 = select_cont()
                 MatchSpec = match_spec()

              This function can be used to restore an opaque continuation returned by select/3 or
              select/1 if the continuation has passed through external  term  format  (been  sent
              between nodes or stored on disk).

              The  reason  for  this  function  is that continuation terms contain compiled match
              specifications and therefore are invalidated if converted to external term  format.
              Given that the original match specification is kept intact, the continuation can be
              restored, meaning it can once again be  used  in  subsequent  select/1  calls  even
              though it has been stored on disk or on another node.

              For more information and examples, see the ets(3erl) module.

          Note:
              This  function  is  rarely  needed  in  application code. It is used by application
              Mnesia to provide distributed select/3 and select/1 sequences. A normal application
              would  either  use Mnesia or keep the continuation from being converted to external
              format.

              The  reason  for  not  having  an  external  representation   of   compiled   match
              specifications  is  performance.  It  can  be subject to change in future releases,
              while this interface remains for backward compatibility.

       safe_fixtable(Name, Fix) -> ok

              Types:

                 Name = tab_name()
                 Fix = boolean()

              If Fix is true, table Name is fixed (once more) by the calling  process,  otherwise
              the table is released. The table is also released when a fixing process terminates.

              If  many  processes  fix  a table, the table remains fixed until all processes have
              released it or terminated. A reference counter is kept on a per process basis,  and
              N consecutive fixes require N releases to release the table.

              It  is  not guaranteed that calls to first/1, next/2, or select and match functions
              work as expected even if the table is fixed; the limited  support  for  concurrency
              provided  by  the  ets(3erl)  module  is  not  yet provided by Dets. Fixing a table
              currently only disables resizing of the hash list of the table.

              If objects have been added while the table was fixed, the hash list starts to  grow
              when  the table is released, which significantly slows down access to the table for
              a period of time.

       select(Continuation) ->
                 {Selection, Continuation2} |
                 '$end_of_table' |
                 {error, Reason}

              Types:

                 Continuation = Continuation2 = select_cont()
                 Selection = [term()]
                 Reason = term()

              Applies a match specification to some objects stored in a table and returns a  non-
              empty  list  of  the results. The table, the match specification, and the number of
              objects that are matched are all defined by Continuation, which is  returned  by  a
              previous call to select/1 or select/3.

              When all objects of the table have been matched, '$end_of_table' is returned.

       select(Name, MatchSpec) -> Selection | {error, Reason}

              Types:

                 Name = tab_name()
                 MatchSpec = match_spec()
                 Selection = [term()]
                 Reason = term()

              Returns  the  results  of  applying  match  specification  MatchSpec to all or some
              objects stored in table Name. The order of the objects  is  not  specified.  For  a
              description of match specifications, see the ERTS User's Guide.

              If  the  keypos'th  element  of  MatchSpec  is  unbound, the match specification is
              applied to all objects of the table. If the keypos'th element is bound,  the  match
              specification is applied to the objects with the correct key(s) only.

              Using  the select functions for traversing all objects of a table is more efficient
              than calling first/1 and next/2 or slot/2.

       select(Name, MatchSpec, N) ->
                 {Selection, Continuation} |
                 '$end_of_table' |
                 {error, Reason}

              Types:

                 Name = tab_name()
                 MatchSpec = match_spec()
                 N = default | integer() >= 0
                 Continuation = select_cont()
                 Selection = [term()]
                 Reason = term()

              Returns the results of applying  match  specification  MatchSpec  to  some  or  all
              objects  stored  in  table  Name.  The order of the objects is not specified. For a
              description of match specifications, see the ERTS User's Guide.

              A tuple of the results of applying the match specification and  a  continuation  is
              returned, unless the table is empty, in which case '$end_of_table' is returned. The
              continuation is to be used when matching more objects by calling select/1.

              If the keypos'th element of MatchSpec is bound, the match specification is  applied
              to  all  objects  of the table with the correct key(s). If the keypos'th element of
              MatchSpec is unbound, the match specification is applied  to  all  objects  of  the
              table,  N  objects  at  a time, until at least one object matches or the end of the
              table is reached. The default, indicated by giving N the value default, is  to  let
              the  number of objects vary depending on the sizes of the objects. All objects with
              the same key are always handled at the same time,  which  implies  that  the  match
              specification can be applied to more than N objects.

              The  table is always to be protected using safe_fixtable/2 before calling select/3,
              otherwise errors can occur when calling select/1.

       select_delete(Name, MatchSpec) -> N | {error, Reason}

              Types:

                 Name = tab_name()
                 MatchSpec = match_spec()
                 N = integer() >= 0
                 Reason = term()

              Deletes each  object  from  table  Name  such  that  applying  match  specification
              MatchSpec   to   the  object  returns  value  true.  For  a  description  of  match
              specifications, see the ERTS User's Guide. Returns the number of deleted objects.

              If the keypos'th element of MatchSpec is bound, the match specification is  applied
              to the objects with the correct key(s) only.

       slot(Name, I) -> '$end_of_table' | Objects | {error, Reason}

              Types:

                 Name = tab_name()
                 I = integer() >= 0
                 Objects = [object()]
                 Reason = term()

              The objects of a table are distributed among slots, starting with slot 0 and ending
              with slot n. Returns the list of  objects  associated  with  slot  I.  If  I  >  n,
              '$end_of_table' is returned.

       sync(Name) -> ok | {error, Reason}

              Types:

                 Name = tab_name()
                 Reason = term()

              Ensures  that all updates made to table Name are written to disk. This also applies
              to tables that have been opened with flag ram_file set to true. In this  case,  the
              contents of the RAM file are flushed to disk.

              Notice  that the space management data structures kept in RAM, the buddy system, is
              also written to the disk. This can take some time if the table is fragmented.

       table(Name) -> QueryHandle

       table(Name, Options) -> QueryHandle

              Types:

                 Name = tab_name()
                 Options = Option | [Option]
                 Option = {n_objects, Limit} | {traverse, TraverseMethod}
                 Limit = default | integer() >= 1
                 TraverseMethod = first_next | select | {select, match_spec()}
                 QueryHandle = qlc:query_handle()

              Returns a Query  List  Comprehension  (QLC)  query  handle.  The  qlc(3erl)  module
              provides a query language aimed mainly for Mnesia, but ETS tables, Dets tables, and
              lists are also recognized by qlc as sources of data. Calling dets:table/1,2 is  the
              means to make Dets table Name usable to qlc.

              When there are only simple restrictions on the key position, qlc uses dets:lookup/2
              to look up the keys. When that is not  possible,  the  whole  table  is  traversed.
              Option traverse determines how this is done:

                * first_next  -  The table is traversed one key at a time by calling dets:first/1
                  and dets:next/2.

                * select - The table is traversed by  calling  dets:select/3  and  dets:select/1.
                  Option  n_objects determines the number of objects returned (the third argument
                  of select/3). The match specification (the  second  argument  of  select/3)  is
                  assembled by qlc:

                  * Simple filters are translated into equivalent match specifications.

                  * More  complicated filters must be applied to all objects returned by select/3
                    given a match specification that matches all objects.

                * {select,  match_spec()} - As for select, the  table  is  traversed  by  calling
                  dets:select/3 and dets:select/1. The difference is that the match specification
                  is specified explicitly. This is how to state match specifications that  cannot
                  easily be expressed within the syntax provided by qlc.

              The following example uses an explicit match specification to traverse the table:

              1> dets:open_file(t, []),
              ok = dets:insert(t, [{1,a},{2,b},{3,c},{4,d}]),
              MS = ets:fun2ms(fun({X,Y}) when (X > 1) or (X < 5) -> {Y} end),
              QH1 = dets:table(t, [{traverse, {select, MS}}]).

              An example with implicit match specification:

              2> QH2 = qlc:q([{Y} || {X,Y} <- dets:table(t), (X > 1) or (X < 5)]).

              The  latter  example  is  equivalent  to  the  former,  which can be verified using
              function qlc:info/1:

              3> qlc:info(QH1) =:= qlc:info(QH2).
              true

              qlc:info/1 returns information  about  a  query  handle.  In  this  case  identical
              information is returned for the two query handles.

       to_ets(Name, EtsTab) -> EtsTab | {error, Reason}

              Types:

                 Name = tab_name()
                 EtsTab = ets:tab()
                 Reason = term()

              Inserts  the objects of the Dets table Name into the ETS table EtsTab. The order in
              which the objects are inserted is not specified. The existing objects  of  the  ETS
              table are kept unless overwritten.

       traverse(Name, Fun) -> Return | {error, Reason}

              Types:

                 Name = tab_name()
                 Fun = fun((Object) -> FunReturn)
                 Object = object()
                 FunReturn =
                     continue | {continue, Val} | {done, Value} | OtherValue
                 Return = [term()] | OtherValue
                 Val = Value = OtherValue = Reason = term()

              Applies  Fun  to  each  object  stored  in  table  Name  in some unspecified order.
              Different actions are taken depending on the return value of Fun. The following Fun
              return values are allowed:

                continue:
                  Continue  to  perform the traversal. For example, the following function can be
                  used to print the contents of a table:

                fun(X) -> io:format("~p~n", [X]), continue end.

                {continue, Val}:
                  Continue the traversal and accumulate Val. The following function  is  supplied
                  to collect all objects of a table in a list:

                fun(X) -> {continue, X} end.

                {done, Value}:
                  Terminate the traversal and return [Value | Acc].

              Any other value OtherValue returned by Fun terminates the traversal and is returned
              immediately.

       update_counter(Name, Key, Increment) -> Result

              Types:

                 Name = tab_name()
                 Key = term()
                 Increment = {Pos, Incr} | Incr
                 Pos = Incr = Result = integer()

              Updates the object with key Key stored in table Name of type set by adding Incr  to
              the  element  at  the  Pos:th  position.  The  new counter value is returned. If no
              position is specified, the element directly following the key is updated.

              This functions provides a way of updating a counter, without having to look  up  an
              object,  update  the  object  by  incrementing an element, and insert the resulting
              object into the table again.

SEE ALSO

       ets(3erl), mnesia(3erl), qlc(3erl)