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Commit4496020

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Avoid reltuples distortion in very small tables.
Consistently avoid trusting a sample of only one page at the point thatVACUUM determines a new reltuples for the target table (though only whenthe table is larger than a single page). This is follow-up work tocommit74388a1, which added a heuristic to prevent reltuples frombecoming distorted by successive VACUUM operations that each scan only asingle heap page (which was itself more or less a bugfix for an issue incommit44fa848, which simplified VACUUM's handling of scanned pages).The original bugfix commit did not account for certain remaining casesthat where not affected by its "2% of total relpages" heuristic. Thishappened with relations that are small enough that just one of its pagesexceeded the 2% threshold, yet still big enough for VACUUM to deemskipping most of its pages via the visibility map worthwhile. reltuplescould still become distorted over time with such a table, at least inscenarios where the VACUUM command is run repeatedly and without thetable itself ever changing.Author: Peter Geoghegan <pg@bowt.ie>Discussion:https://postgr.es/m/CAH2-Wzk7d4m3oEbEWkWQKd+gz-eD_peBvdXVk1a_KBygXadFeg@mail.gmail.comBackpatch: 15-, where the rules for scanned pages changed.
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‎src/backend/commands/vacuum.c‎

Lines changed: 10 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -1234,31 +1234,25 @@ vac_estimate_reltuples(Relation relation,
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if (scanned_pages >=total_pages)
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returnscanned_tuples;
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1237-
/*
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* If scanned_pages is zero but total_pages isn't, keep the existing value
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* of reltuples. (Note: we might be returning -1 in this case.)
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*/
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if (scanned_pages==0)
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returnold_rel_tuples;
1243-
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/*
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* When successive VACUUM commands scan the same few pages again and
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* again, without anything from the table really changing, there is a risk
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* that our beliefs about tuple density will gradually become distorted.
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* It's particularly important to avoid becoming confused in this way due
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* to vacuumlazy.c implementation details. For example, the tendency for
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* our caller to always scan the last heap page should not ever cause us
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* to believe that every page in the table must be just like the last
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* page.
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* This might be caused by vacuumlazy.c implementation details, such as
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* its tendency to always scan the last heap page. Handle that here.
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*
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* If the relation is _exactly_ the same size according to the existing
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* pg_class entry, and only a few of its pages (less than 2%) were
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* scanned, keep the existing value of reltuples. Also keep the existing
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* value when only a subset of rel's pages <= a single page were scanned.
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*
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* We apply a heuristic to avoid these problems: if the relation is
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* exactly the same size as it was at the end of the last VACUUM, and only
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* a few of its pages (less than a quasi-arbitrary threshold of 2%) were
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* scanned by this VACUUM, assume that reltuples has not changed at all.
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* (Note: we might be returning -1 here.)
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*/
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if (old_rel_pages==total_pages&&
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scanned_pages< (double)total_pages*0.02)
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returnold_rel_tuples;
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if (scanned_pages <=1)
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returnold_rel_tuples;
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/*
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* If old density is unknown, we can't do much except scale up

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