Detailed comparison and analysis of the lock mechanism of VACUUM operation in PostgreSQL

Comparison of locking mechanisms for VACUUM operations in PostgreSQL

PostgreSQL provides three main VACUUM operations: AutoVACUUM, VACUUM, and VACUUM FULL, which differ significantly in locking mechanisms. Here is a detailed comparison of them:

Lock mechanism comparison table

Operation Type	Main lock types	Blocking situation	Concurrency impact	Recommended usage scenarios
AutoVACUUM	ShareUpdateExclusiveLock	Block only DDL operations	Minimal impact, almost no blocking DML	Regular maintenance of production environment
VACUUM	ShareUpdateExclusiveLock	Block only DDL operations	Small impact, no blocking DML	Manually trigger maintenance or table-specific optimization
VACUUM FULL	AccessExclusiveLock	Block all operations (DDL and DML)	Completely exclusive table	Special situations where large amounts of space are needed

Detailed analysis

1 AutoVACUUM

Lock features：

Table-level lock:ShareUpdateExclusiveLock(Level 4 Lock)
Row-level lock: briefly acquire exclusive locks when cleaning dead tuples

Behavioral characteristics：

SELECT/INSERT/UPDATE/DELETE operation will not be blocked
Will block DDL operations such as ALTER TABLE, DROP TABLE, etc.
Automatically execute by the background process and automatically adjust the speed according to the system load

Monitoring commands：

SELECT query, wait_event_type, wait_event 
FROM pg_stat_activity 
WHERE query LIKE '%autovacuum%';

2 Normal VACUUM

Lock features：

Table-level lock:ShareUpdateExclusiveLock(Same as AutoVACUUM)
Row-level lock: Similar to AutoVACUUM

Differences from AutoVACUUM：

Manual execution, can control execution time and parameters
Can be executed for specific tables or databases
You can add VERBOSE option to view details

Sample command：

VACUUM (VERBOSE, ANALYZE) my_table;

3 VACUUM FULL

Lock features：

Table-level lock:AccessExclusiveLock(Level 8 lock, highest level)
Completely rewrite table files, requiring exclusive access

Behavioral characteristics：

Block all access to the table (including SELECT)
Will rewrite the entire table file and recycle more space
May cause long-term unavailability of services

Risk warning：

-- Use with caution in production environment！
VACUUM FULL my_table;

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