From: | "Matthew T(dot) O'Connor" <matthew(at)zeut(dot)net> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, pgsql-hackers(at)postgreSQL(dot)org |
Subject: | Re: Autovacuum versus rolled-back transactions |
Date: | 2007-06-01 13:03:51 |
Message-ID: | 46601937.2090307@zeut.net |
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Lists: | pgsql-hackers |
Tom Lane wrote:
> ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> writes:
>> Our documentation says
>> | analyze threshold = analyze base threshold
>> | + analyze scale factor * number of tuples
>> | is compared to the total number of tuples inserted, updated, or deleted
>> | since the last ANALYZE.
>
>> but deleted tuples are not considered in the total number, because the delta
>> of {n_live_tuples + n_dead_tuples} is not changed by DELETE. We add the number
>> of DELETE into n_live_tuples and subtract it from n_dead_tuples.
>
> Yeah, I was concerned about that when I was making the patch, but didn't
> see any simple fix. A large number of DELETEs (without any inserts or
> updates) would trigger a VACUUM but not an ANALYZE, which in the worst
> case would be bad because the stats could have shifted.
>
> We could fix this at the cost of carrying another per-table counter in
> the stats info, but I'm not sure it's worth it.
I believe that whenever autovacuum performs a VACUUM it actually
performs a VACUUM ANALYZE at leas the old contrib version did and I
think Alvaro copied that.
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