Re: select slows from 3 seconds to 30 seconds

Gary Stainburn <gary(dot)stainburn(at)ringways(dot)co(dot)uk> writes:
> I have the query below which when run takes approx 3 seconds. However when I
> add the condition 'and w_ws_id = 10' onto the end changes to 30+ seconds.
> Can anyone see why? I've included the explain for the long select.

You really ought to provide EXPLAIN ANALYZE output for both versions if
you want intelligent commentary --- otherwise we're just guessing.

But I would guess the problem is that the planner is way off about the
number of rows satisfying the joint condition --- it thinks two:

> -> Index Scan using walon_hide_index on walon (cost=0.00..85.04 rows=2 width=140)
> Index Cond: (w_hide = 0)
> Filter: (w_ws_id = 10)

This plan is good if there really are only a couple rows, but degrades
quickly if there are many...

regards, tom lane