Ubuntu Manpage: REINDEX - rebuild indexes

Provided by: postgresql-client-12_12.22-0ubuntu0.20.04.4_amd64

NAME

       REINDEX - rebuild indexes

SYNOPSIS

       REINDEX [ ( VERBOSE ) ] { INDEX | TABLE | SCHEMA | DATABASE | SYSTEM } [ CONCURRENTLY ] name

DESCRIPTION

       REINDEX rebuilds an index using the data stored in the index's table, replacing the old copy of the
       index. There are several scenarios in which to use REINDEX:

       •   An index has become corrupted, and no longer contains valid data. Although in theory this should
           never happen, in practice indexes can become corrupted due to software bugs or hardware failures.
           REINDEX provides a recovery method.

       •   An index has become “bloated”, that is it contains many empty or nearly-empty pages. This can occur
           with B-tree indexes in PostgreSQL under certain uncommon access patterns.  REINDEX provides a way to
           reduce the space consumption of the index by writing a new version of the index without the dead
           pages. See Section 24.2 for more information.

       •   You have altered a storage parameter (such as fillfactor) for an index, and wish to ensure that the
           change has taken full effect.

       •   If an index build fails with the CONCURRENTLY option, this index is left as “invalid”. Such indexes
           are useless but it can be convenient to use REINDEX to rebuild them. Note that only REINDEX INDEX is
           able to perform a concurrent build on an invalid index.

PARAMETERS

INDEX
Recreate the specified index.

TABLE
Recreate all indexes of the specified table. If the table has a secondary “TOAST” table, that is
reindexed as well.

SCHEMA
Recreate all indexes of the specified schema. If a table of this schema has a secondary “TOAST”
table, that is reindexed as well. Indexes on shared system catalogs are also processed. This form of
REINDEX cannot be executed inside a transaction block.

DATABASE
Recreate all indexes within the current database. Indexes on shared system catalogs are also
processed. This form of REINDEX cannot be executed inside a transaction block.

SYSTEM
Recreate all indexes on system catalogs within the current database. Indexes on shared system
catalogs are included. Indexes on user tables are not processed. This form of REINDEX cannot be
executed inside a transaction block.

name
The name of the specific index, table, or database to be reindexed. Index and table names can be
schema-qualified. Presently, REINDEX DATABASE and REINDEX SYSTEM can only reindex the current
database, so their parameter must match the current database's name.

CONCURRENTLY
When this option is used, PostgreSQL will rebuild the index without taking any locks that prevent
concurrent inserts, updates, or deletes on the table; whereas a standard index rebuild locks out
writes (but not reads) on the table until it's done. There are several caveats to be aware of when
using this option — see Rebuilding Indexes Concurrently.

For temporary tables, REINDEX is always non-concurrent, as no other session can access them, and
non-concurrent reindex is cheaper.

VERBOSE
Prints a progress report as each index is reindexed.

NOTES

If you suspect corruption of an index on a user table, you can simply rebuild that index, or all indexes
on the table, using REINDEX INDEX or REINDEX TABLE.

Things are more difficult if you need to recover from corruption of an index on a system table. In this
case it's important for the system to not have used any of the suspect indexes itself. (Indeed, in this
sort of scenario you might find that server processes are crashing immediately at start-up, due to
reliance on the corrupted indexes.) To recover safely, the server must be started with the -P option,
which prevents it from using indexes for system catalog lookups.

One way to do this is to shut down the server and start a single-user PostgreSQL server with the -P
option included on its command line. Then, REINDEX DATABASE, REINDEX SYSTEM, REINDEX TABLE, or REINDEX
INDEX can be issued, depending on how much you want to reconstruct. If in doubt, use REINDEX SYSTEM to
select reconstruction of all system indexes in the database. Then quit the single-user server session and
restart the regular server. See the postgres(1) reference page for more information about how to interact
with the single-user server interface.

Alternatively, a regular server session can be started with -P included in its command line options. The
method for doing this varies across clients, but in all libpq-based clients, it is possible to set the
PGOPTIONS environment variable to -P before starting the client. Note that while this method does not
require locking out other clients, it might still be wise to prevent other users from connecting to the
damaged database until repairs have been completed.

REINDEX is similar to a drop and recreate of the index in that the index contents are rebuilt from
scratch. However, the locking considerations are rather different. REINDEX locks out writes but not
reads of the index's parent table. It also takes an ACCESS EXCLUSIVE lock on the specific index being
processed, which will block reads that attempt to use that index. In contrast, DROP INDEX momentarily
takes an ACCESS EXCLUSIVE lock on the parent table, blocking both writes and reads. The subsequent CREATE
INDEX locks out writes but not reads; since the index is not there, no read will attempt to use it,
meaning that there will be no blocking but reads might be forced into expensive sequential scans.

Reindexing a single index or table requires being the owner of that index or table. Reindexing a schema
or database requires being the owner of that schema or database. Note that is therefore sometimes
possible for non-superusers to rebuild indexes of tables owned by other users. However, as a special
exception, when REINDEX DATABASE, REINDEX SCHEMA or REINDEX SYSTEM is issued by a non-superuser, indexes
on shared catalogs will be skipped unless the user owns the catalog (which typically won't be the case).
Of course, superusers can always reindex anything.

Reindexing partitioned tables or partitioned indexes is not supported. Each individual partition can be
reindexed separately instead.

Rebuilding Indexes Concurrently
Rebuilding an index can interfere with regular operation of a database. Normally PostgreSQL locks the
table whose index is rebuilt against writes and performs the entire index build with a single scan of the
table. Other transactions can still read the table, but if they try to insert, update, or delete rows in
the table they will block until the index rebuild is finished. This could have a severe effect if the
system is a live production database. Very large tables can take many hours to be indexed, and even for
smaller tables, an index rebuild can lock out writers for periods that are unacceptably long for a
production system.

PostgreSQL supports rebuilding indexes with minimum locking of writes. This method is invoked by
specifying the CONCURRENTLY option of REINDEX. When this option is used, PostgreSQL must perform two
scans of the table for each index that needs to be rebuilt and wait for termination of all existing
transactions that could potentially use the index. This method requires more total work than a standard
index rebuild and takes significantly longer to complete as it needs to wait for unfinished transactions
that might modify the index. However, since it allows normal operations to continue while the index is
being rebuilt, this method is useful for rebuilding indexes in a production environment. Of course, the
extra CPU, memory and I/O load imposed by the index rebuild may slow down other operations.

The following steps occur in a concurrent reindex. Each step is run in a separate transaction. If there
are multiple indexes to be rebuilt, then each step loops through all the indexes before moving to the
next step.

1. A new transient index definition is added to the catalog pg_index. This definition will be used to
replace the old index. A SHARE UPDATE EXCLUSIVE lock at session level is taken on the indexes being
reindexed as well as their associated tables to prevent any schema modification while processing.

2. A first pass to build the index is done for each new index. Once the index is built, its flag
pg_index.indisready is switched to “true” to make it ready for inserts, making it visible to other
sessions once the transaction that performed the build is finished. This step is done in a separate
transaction for each index.

3. Then a second pass is performed to add tuples that were added while the first pass was running. This
step is also done in a separate transaction for each index.

4. All the constraints that refer to the index are changed to refer to the new index definition, and the
names of the indexes are changed. At this point, pg_index.indisvalid is switched to “true” for the
new index and to “false” for the old, and a cache invalidation is done causing all sessions that
referenced the old index to be invalidated.

5. The old indexes have pg_index.indisready switched to “false” to prevent any new tuple insertions,
after waiting for running queries that might reference the old index to complete.

6. The old indexes are dropped. The SHARE UPDATE EXCLUSIVE session locks for the indexes and the table
are released.

If a problem arises while rebuilding the indexes, such as a uniqueness violation in a unique index, the
REINDEX command will fail but leave behind an “invalid” new index in addition to the pre-existing one.
This index will be ignored for querying purposes because it might be incomplete; however it will still
consume update overhead. The psql \d command will report such an index as INVALID:

postgres=# \d tab
Table "public.tab"
Column | Type | Modifiers
--------+---------+-----------
col | integer |
Indexes:
"idx" btree (col)
"idx_ccnew" btree (col) INVALID

If the index marked INVALID is suffixed ccnew, then it corresponds to the transient index created during
the concurrent operation, and the recommended recovery method is to drop it using DROP INDEX, then
attempt REINDEX CONCURRENTLY again. If the invalid index is instead suffixed ccold, it corresponds to the
original index which could not be dropped; the recommended recovery method is to just drop said index,
since the rebuild proper has been successful.

Regular index builds permit other regular index builds on the same table to occur simultaneously, but
only one concurrent index build can occur on a table at a time. In both cases, no other types of schema
modification on the table are allowed meanwhile. Another difference is that a regular REINDEX TABLE or
REINDEX INDEX command can be performed within a transaction block, but REINDEX CONCURRENTLY cannot.

Like any long-running transaction, REINDEX on a table can affect which tuples can be removed by
concurrent VACUUM on any other table.

REINDEX SYSTEM does not support CONCURRENTLY since system catalogs cannot be reindexed concurrently.

Furthermore, indexes for exclusion constraints cannot be reindexed concurrently. If such an index is
named directly in this command, an error is raised. If a table or database with exclusion constraint
indexes is reindexed concurrently, those indexes will be skipped. (It is possible to reindex such indexes
without the CONCURRENTLY option.)

EXAMPLES

       Rebuild a single index:

           REINDEX INDEX my_index;

       Rebuild all the indexes on the table my_table:

           REINDEX TABLE my_table;

       Rebuild all indexes in a particular database, without trusting the system indexes to be valid already:

           $ export PGOPTIONS="-P"
           $ psql broken_db
           ...
           broken_db=> REINDEX DATABASE broken_db;
           broken_db=> \q

       Rebuild indexes for a table, without blocking read and write operations on involved relations while
       reindexing is in progress:

           REINDEX TABLE CONCURRENTLY my_broken_table;

COMPATIBILITY

       There is no REINDEX command in the SQL standard.