Date: Tue, 24 Mar 2020 23:46:51 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	Andres Freund <andres@anarazel.de>,
	Michael Paquier <michael@paquier.xyz>, pgsql-hackers@postgresql.org
Subject: Re: error context for vacuum to include block number
Message-ID: <20200325044651.GO21443@telsasoft.com>
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On Wed, Mar 25, 2020 at 01:34:43PM +0900, Masahiko Sawada wrote:
> I meant that with the patch, suppose that the table has 100 blocks and
> we're truncating it to 50 blocks in RelationTruncate(), the error
> context message will be "while truncating relation "aaa.bbb" to 100
> blocks", which is not correct.

> I think it should be "while truncating
> relation "aaa.bbb" to 50 blocks". We can know the relation can be
> truncated to 50 blocks by the result of count_nondeletable_pages(). So
> if we update the arguments before it we will use the number of blocks
> of relation before truncation.

Hm, yea, at that point it's:
|new_rel_pages = RelationGetNumberOfBlocks(onerel);
..so we can do better.

> My suggestion is either that we change the error message to, for
> example, "while truncating relation "aaa.bbb" having 100 blocks", or
> that we change the patch so that we can use "50 blocks" in the error
> context message.

We could do:

        update_vacuum_error_cbarg(vacrelstats,
				  VACUUM_ERRCB_PHASE_TRUNCATE,
				  InvalidBlockNumber, NULL, false);

        new_rel_pages = count_nondeletable_pages(onerel, vacrelstats);
        vacrelstats->blkno = new_rel_pages;

...

        case VACUUM_ERRCB_PHASE_TRUNCATE:
            if (BlockNumberIsValid(cbarg->blkno))
                errcontext("while truncating relation \"%s.%s\" to %u blocks",
                           cbarg->relnamespace, cbarg->relname, cbarg->blkno);
            else
                /* Error happened before/during count_nondeletable_pages() */
                errcontext("while truncating relation \"%s.%s\"",
                           cbarg->relnamespace, cbarg->relname);
            break;

-- 
Justin