On Fri, Feb 17, 2017 at 3:49 PM, Tomas Vondra <tomas.von...@2ndquadrant.com> wrote:
> That may seem a bit strange, but I'd bet it finds the short value in some > statistic (MCV, histogram) ans so can provide very accurate estimate. -> Index Only Scan using tab_idx1 on tab (cost=0.27..8.29 rows=1 width=0) (actual time=0.043..0.043 rows=0 loops=1) I'm not seeing how any of the statistic columns would capture a value that doesn't actually appear in the table...(actual ... row=0) Unless there is some prefix matching going on here since the short value is a substring(1, n) of the longer one which does appear 5 times. I guess maybe because the value doesn't appear it uses the index (via IOS) to confirm absence (or near absence, i.e., 1) while, knowing the larger value appears 5 times out of 223, it decides a quick table scan is faster than any form of double-lookup (whether on the visibility map or the heap). https://www.postgresql.org/docs/9.6/static/indexes-index-only-scans.html David J.
