Re: 100x slowdown for nearly identical tables

Craig James <cjames(at)emolecules(dot)com> writes:
> I have two tables that are nearly identical, yet the same query runs 100x
> slower on the newer one. ...

> db=> explain analyze select id, 1 from str_conntab
> where (id >= 12009977 and id <= 12509976) order by id;

> Index Scan using new_str_conntab_pkey_3217 on str_conntab
> (cost=0.00..230431.33 rows=87827 width=4)
> (actual time=65.771..51341.899 rows=48613 loops=1)
> Index Cond: ((id >= 12009977) AND (id <= 12509976))
> Total runtime: 51350.556 ms

> db=> explain analyze select id, 1 from old_str_conntab
> where (id >= 12009977 and id <= 12509976) order by id;

> Index Scan using str_conntab_pkey on old_str_conntab
> (cost=0.00..82262.56 rows=78505 width=4)
> (actual time=38.327..581.235 rows=48725 loops=1)
> Index Cond: ((id >= 12009977) AND (id <= 12509976))
> Total runtime: 586.071 ms

It looks like old_str_conntab is more or less clustered by "id",
and str_conntab not so much. You could try EXPLAIN (ANALYZE, BUFFERS)
(on newer PG versions) to verify how many distinct pages are getting
touched during the indexscan.

regards, tom lane