Re: Joining 16 tables seems slow

[Pavel Stehule <pavel.stehule@xxxxxxxxx>]

hi

2017-09-12 8:45 GMT+02:00 Frank Millman <frank@xxxxxxxxxxxx>:

Hi all

 

I am using 9.4.4 on Fedora 22.

 

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.

 

My test involves selecting a single row. Both versions work. The first version takes 0.06 seconds. The second takes 0.23 seconds. On further experimentation, the time for the second one seems to taken in setting up the joins, because if I omit selecting anything from the joined tables, it still takes 0.23 seconds.

 

Exactly the same exercise on Sql Server results in 0.06 seconds for both versions.

 

I realise that, if I was selecting a large number of rows, 0.23 seconds is trivial and the overall result could be different. But still, it seems odd.

 

Is this normal, or should I investigate further?

please send result of explain analyze

you can experimentally try increase FROM_COLLAPSE_LIMIT to some higher number 14 maybe 16

regards

 

 

Frank Millman