Re: [Proposal] Allow users to specify multiple tables in VACUUM commands

From: "Bossart, Nathan" <bossartn(at)amazon(dot)com>
To: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Subject: Re: [Proposal] Allow users to specify multiple tables in VACUUM commands
Date: 2017-05-16 22:56:55
Message-ID: 3805C3A5-9C3A-4581-BCB9-2C7305095E3E@amazon.com
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I’ve attached a v3 patch that addresses your feedback:

Changes:
- removed extra square brackets in documentation changes
- removed unnecessary documentation changes for parameter list
- eliminated one of the new node types
- renamed ‘rel’ argument to ‘relations’ in vacuum(…)
- moved relations list to vacuum memory context in vacuum(…)
- minor addition to VACUUM regression test
- rebased with master

On 5/15/17, 11:00 PM, "Michael Paquier" <michael(dot)paquier(at)gmail(dot)com> wrote:
> Hm. If multiple tables are specified and that some of them take a long
> time, it could be possible that an error still happens if the
> definition of one of those tables changes while VACUUM is in the
> middle of running. And this makes moot the error checks that happened
> at first step. So it seems to me that we definitely should have a
> WARNING if multiple tables are defined anyway, and that to avoid code
> duplication we may want to just do those checks once, before
> processing one of the listed tables. It is true that is would be easy
> to miss a WARNING in the VERBOSE logs, but issuing an ERROR would
> really be frustrating in the middle of a nightly run of VACUUM.

I think this issue already exists, as this comment in get_rel_oids(…) seems to indicate:

/*
* Since we don't take a lock here, the relation might be gone, or the
* RangeVar might no longer refer to the OID we look up here. In the
* former case, VACUUM will do nothing; in the latter case, it will
* process the OID we looked up here, rather than the new one. Neither
* is ideal, but there's little practical alternative, since we're
* going to commit this transaction and begin a new one between now
* and then.
*/
relid = RangeVarGetRelid(vacrel, NoLock, false);

With the patch applied, I believe this statement still holds true. So if the relation disappears before we get to vacuum_rel(…), we will simply skip it and move on to the next one. The vacuum_rel(…) code provides a WARNING in many cases (e.g. the user does not have privileges to VACUUM the table), but we seem to silently skip the table when it disappears before the call to vacuum_rel(…). If we added a WARNING to the effect of “skipping vacuum of <table_name> — relation no longer exists” for this case, I think what you are suggesting would be satisfied.

However, ANALYZE has a slight caveat. While analyze_rel(…) silently skips the relation if it no longer exists like vacuum_rel(…) does, we do not pre-validate the columns list at all. So, in an ANALYZE statement with multiple tables and columns specified, it’ll only fail once we get to the undefined column. To fix this, we could add a check for the column lists near get_rel_oids(…) and adjust do_analyze_rel(…) to emit a WARNING and skip any columns that vanish in the meantime.

Does this seem like a sane approach?

1. Emit WARNING when skipping if relation disappears before we get to it.
2. Early in vacuum(…), check that the specified columns exist.
3. Emit WARNING and skip any specified columns that vanish before processing.

Nathan

Attachment Content-Type Size
vacuum_multiple_tables_v3.patch application/octet-stream 25.6 KB

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