If you suspect corruption of an index on a user table, you can simply rebuild that index, or all indexes
on the table, using REINDEXINDEX or REINDEXTABLE.
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, REINDEXDATABASE, REINDEXSYSTEM, REINDEXTABLE, or REINDEXINDEX can be issued, depending on how much you want to reconstruct. If in doubt, use REINDEXSYSTEM 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 particular, the query planner tries
to take an ACCESS SHARE lock on every index of the table, regardless of the query, and so REINDEX blocks
virtually any queries except for some prepared queries whose plan has been cached and which don't use
this very index. In contrast, DROPINDEX momentarily takes an ACCESS EXCLUSIVE lock on the parent table,
blocking both writes and reads. The subsequent CREATEINDEX 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.
While REINDEX is running, the search_path is temporarily changed to pg_catalog, pg_temp.
Reindexing a single index or table requires having the MAINTAIN privilege on the table. Note that while
REINDEX on a partitioned index or table requires having the MAINTAIN privilege on the partitioned table,
such commands skip the privilege checks when processing the individual partitions. Reindexing a schema or
database requires being the owner of that schema or database or having privileges of the pg_maintain
role. Note specifically that it's thus possible for non-superusers to rebuild indexes of tables owned by
other users. However, as a special exception, REINDEXDATABASE, REINDEXSCHEMA, and REINDEXSYSTEM will
skip indexes on shared catalogs unless the user has the MAINTAIN privilege on the catalog.
Reindexing partitioned indexes or partitioned tables is supported with REINDEXINDEX or REINDEXTABLE,
respectively. Each partition of the specified partitioned relation is reindexed in a separate
transaction. Those commands cannot be used inside a transaction block when working on a partitioned table
or index.
When using the TABLESPACE clause with REINDEX on a partitioned index or table, only the tablespace
references of the leaf partitions are updated. As partitioned indexes are not updated, it is recommended
to separately use ALTERTABLEONLY on them so as any new partitions attached inherit the new tablespace.
On failure, it may not have moved all the indexes to the new tablespace. Re-running the command will
rebuild all the leaf partitions and move previously-unprocessed indexes to the new tablespace.
If SCHEMA, DATABASE or SYSTEM is used with TABLESPACE, system relations are skipped and a single WARNING
will be generated. Indexes on TOAST tables are rebuilt, but not moved to the new tablespace.
RebuildingIndexesConcurrently
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 REINDEXCONCURRENTLY 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 REINDEXTABLE or
REINDEXINDEX command can be performed within a transaction block, but REINDEXCONCURRENTLY cannot.
Like any long-running transaction, REINDEX on a table can affect which tuples can be removed by
concurrent VACUUM on any other table.
REINDEXSYSTEM 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.)
Each backend running REINDEX will report its progress in the pg_stat_progress_create_index view. See
Section 27.4.4 for details.
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