Re: [GENERAL] Joining 16 tables seems slow

 

2017-09-12 12:39 GMT+02:00 Pavel Stehule <pavel.stehule@gmail.com>:

 

 

2017-09-12 12:25 GMT+02:00 Frank Millman <frank@chagford.com>:

Pavel Stehule wrote:

 

2017-09-12 9:36 GMT+02:00 Frank Millman <frank@chagford.com>:

Pavel Stehule wrote:

>

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

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.

>

> please send result of explain analyze

>

> 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.

 

I have attached files containing my SQL command, and the results of EXPLAIN ANALYSE

 

> please use https://explain.depesz.com/ for both plans (slow, fast)

 

 

Here are the results -

 

sql_slow - https://explain.depesz.com/s/9vn3

 

sql_fast - https://explain.depesz.com/s/oW0F

 

I don't see any issue there - it looks like some multi dimensional query and it should not be well optimized due not precious estimations. The slow query has much more complex - some bigger logic is under nested loop - where estimation is not fully correct, probably due dependencies between columns.

 

what does SET enable_nestloop to off;

>

> from statistics - the ar_tran_inv table is scanned 6x in slow query and 2times in fast query. Maybe there should be some index

>

 

Setting enable_nestloop to off makes no difference.

 

Setting from_collapse_limit and join_collapse_limit to 16, as suggested by Tom, actually slowed it down.

 

I mentioned before that I was running this from python, which complicated it slightly. I have now saved the command to a file on the Fedora side, so I can execute it in psql using the ‘\i’ command. It makes life easier, and I can use ‘\timing’ to time it. It shows exactly the same results.

 

It could be an index problem, but I have just double-checked that, if I remove the lines from the body of the statement that actually select from the joined tables, it makes virtually no difference. However, maybe the planner checks to see what indexes it has before preparing the query, so that does not rule it out as a possibility.

 

I will play with it some more tomorrow, when my  brain is a bit fresher. I will report back with any results.

 

Frank