Re: Number of characters in column preventing index usage

On Fri, Feb 17, 2017 at 3:49 PM, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)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.​