| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "Frank Millman" <frank(at)chagford(dot)com> |
| Cc: | pgsql-general(at)postgresql(dot)org |
| Subject: | Re: Joining 16 tables seems slow |
| Date: | 2017-09-12 12:01:02 |
| Message-ID: | 31084.1505217662@sss.pgh.pa.us |
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| Lists: | pgsql-general |
"Frank Millman" <frank(at)chagford(dot)com> writes:
> Pavel Stehule wrote:
>> 2017-09-12 8:45 GMT+02:00 Frank Millman <frank(at)chagford(dot)com>:
>>> I am experimenting with optimising a SQL statement. One version uses 4 LEFT JOIN’s and a 5-way CASE statement in the body. The second moves the filtering into the JOIN section, and I end up with 16 LEFT JOIN’s and no CASE statements.
>> you can experimentally try increase FROM_COLLAPSE_LIMIT to some higher number 14 maybe 16
> I tried increasing FROM_COLLAPSE_LIMIT, but it made no difference.
For this you need to increase join_collapse_limit, not
from_collapse_limit. (Usually, though, there's little reason not to keep
them the same.)
regards, tom lane
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