2016-09-09 22:41 GMT+09:00 Tom Lane <t...@sss.pgh.pa.us>: > Satoshi Nagayasu <sn...@uptime.jp> writes: >> According to the manual, running REINDEX does not take any locks >> on the parent table which block read operations. >> Actually, REINDEX blocks SELECT queries, maybe in the planning phase. > > Hm. REINDEX does take out only ShareLock on the table, which would not > block DML, but it takes out AccessExclusiveLock on the index. That > blocks the planner's attempts to acquire information about the table's > indexes.
Sorry if I didn't explain my consideration well. In short, I would like to say, "REINDEX TABLE pgbench_accounts" would block "SELECT count(*) FROM pgbench_accounts". That's the situation what many of users don't expect from the manual. For example, SessionA> BEGIN; SessionA> REINDEX TABLE pgbench_accounts; SessionB> SELECT count(*) FROM pgbench_accounts; -- This statement would be blocked by the REINDEX and the locks. Many people understand that "REINDEX does not block read (SELECT) operations" according to the manual. That seems misunderstanding. So, I would like to improve the explanation of REINDEX and locks. At least, I think we should add some explanation about the planning phase would touch the index, and it could be blocked by REINDEX. > In the case of an update query I think there's little we can do about > this; the executor would have to update the index anyway. For a pure > SELECT, you could imagine having the planner do a conditional lock acquire > and ignore the index if that fails. Would that be better than blocking? > Not sure. You could end up with a really bad plan if the index was > critical for efficient processing of the query. Well, I would not intend to modify the implementation for now. I just notice the users that REINDEX could block even SELECT statements in the official documents because it is very critical for web services. Regards, -- Satoshi Nagayasu <sn...@uptime.jp> -- Sent via pgsql-docs mailing list (pgsql-docs@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-docs
