diff options
| -rw-r--r-- | src/backend/commands/vacuum.c | 19 |
1 files changed, 19 insertions, 0 deletions
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c index b6767a5ff8c..50a4a612e58 100644 --- a/src/backend/commands/vacuum.c +++ b/src/backend/commands/vacuum.c @@ -1239,6 +1239,25 @@ vac_estimate_reltuples(Relation relation, return old_rel_tuples; /* + * When successive VACUUM commands scan the same few pages again and + * again, without anything from the table really changing, there is a risk + * that our beliefs about tuple density will gradually become distorted. + * It's particularly important to avoid becoming confused in this way due + * to vacuumlazy.c implementation details. For example, the tendency for + * our caller to always scan the last heap page should not ever cause us + * to believe that every page in the table must be just like the last + * page. + * + * We apply a heuristic to avoid these problems: if the relation is + * exactly the same size as it was at the end of the last VACUUM, and only + * a few of its pages (less than a quasi-arbitrary threshold of 2%) were + * scanned by this VACUUM, assume that reltuples has not changed at all. + */ + if (old_rel_pages == total_pages && + scanned_pages < (double) total_pages * 0.02) + return old_rel_tuples; + + /* * If old density is unknown, we can't do much except scale up * scanned_tuples to match total_pages. */ |