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pl/pgsql functions outperforming sql ones?

From: "Carlo Stonebanks" <stonec(dot)register(at)sympatico(dot)ca>
To: <pgsql-performance(at)postgresql(dot)org>
Subject: pl/pgsql functions outperforming sql ones?
Date: 2012-01-27 00:09:13
Message-ID: CE4615C16F2F41CAB0E67D4099BF20D2@CAPRICA (view raw or flat)
Thread:
Lists: pgsql-performance
Assuming there was some sort of cost to pl/pgsql, I rewrote a bunch of
stored functions s in straight SQL. Each stored proc was calling the next,
so to get the full effect I had to track down all the pl/pgsql stored
functions and convert them to sql. However, I was surprised to find after
all of the rewrites, the LANGUAGE sql procs caused the queries to run slower
than the LANGUAGE plpgsql.

 

None of the stored functions selected from tables, the operated on and
returned scalar values - it was all assign variables, if/then/else - not
even any looping.

 

For those who need the dirty details, here they are. If you happen to think
this behavior is expected, I needn't bore you - just let me know!

 

Thanks,

 

Carlo

 

This was all triggered during the optimization of a query like this:

 

SELECT myVar

FROM myTable

WHERE myFunc(myVar);

 

Looking at EXPLAIN ANALYSE I saw something like this:

 

Filter: myFunc(myVar)

 

I rewrote the body of myFunc(myVar) something like this:

 

SELECT CASE WHEN myVar IS NULL THEN false ELSE myOtherFunc(myVar) END

 

When I reran EXPLAIN ANALYZE I got this:

 

Filter: SELECT CASE WHEN myVar IS NULL THEN false ELSE myOtherFunc(myVar)
END

 

Nice. So, I did the same treatment to myOtherFunc() (converted to straight
sql) but the EXPLAIN ANALYZE didn't change (reasonable, I guess - how deep
would I expect it to go?)

 

All of the procs were IMMUTABLE.

 

I was very surprised to find that the query now ran much slower by a factor
of 4.

 

 

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