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Multixact ID
An identifier used support row locking by multiple transactions
A multixact ID is an internal identifier used support row locking by multiple transactions.
Multixact IDs are created when transactions use "SELECT … FOR UPDATE" (or one of these lock modes: SHARE, KEY SHARE, NO KEY UPDATE) to lock and update tuples.
Multixact IDs live in the pg_multixact directory.
Examples
Show multixact ID usage:
SELECT relname, relminmxid,
$NextMultiXactId-relminmxid::TEXT::BIGINT AS age
FROM pg_class
WHERE relminmxid::TEXT::BIGINT <> 0
ORDER BY relminmxid::TEXT::BIGINT ASC
LIMIT 10;
where $NextMultiXactId is the value returned by SELECT next_multixact_id FROM pg_control_checkpoint() (for PostgreSQL 9.5 and earlier, the value contained in pg_controldata output for "Latest checkpoint's NextMultiXactId").
References
- PostgreSQL documentation: Multixacts and Wraparound
Categories
See also
mxid_age(), vacuum_multixact_freeze_table_age, autovacuum_multixact_freeze_table_age