Chris Kratz wrote:
Hello All,

We are struggling with a specific query that is killing us. When doing explain analyze on the entire query, we *seem* to be getting killed by the estimated number of rows on a case statement calculation.

I've included a snippet from the explain analyze of the much larger query. The line in question, (cost=0.00..106.52 rows=1 width=16) (actual time=0.048..67.728 rows=4725 loops=1) shows that it returned 4700 rows instead of 1 which when coupled with a later join causes the statement to run over 3 minutes.[1] It seems that it thinks that the scan on role_id is going to return 1 row, but in reality returns 4725 rows. The case statement causing the problem uses todays date to see if a particular row is still active. Here is a test case showing how far off the estimate is from the reality. [2]

[2] A much simpler statement triggers the incorrect row counts here.

explain analyze
select * from roles rol where
           WHEN rol.role_id IS NULL
           THEN NULL
           WHEN rol."begin" IS NOT NULL and rol."end" IS NOT NULL
CASE WHEN TIMESTAMP 'now'>=rol."begin" and TIMESTAMP 'now'<=rol."end"
             THEN 'Active'
             ELSE 'Inactive' END
           WHEN rol."begin" IS NOT NULL
             CASE WHEN TIMESTAMP 'now'>=rol."begin"
             THEN 'Active'
             ELSE 'Inactive' END
           WHEN rol."end" IS NOT NULL
             CASE WHEN TIMESTAMP 'now'<=rol."end"
             THEN 'Active'
             ELSE 'Inactive' END
           ELSE 'Active'
           END = 'Active'

Aside #1 - I'm not entirely clear how role_id can be null since you seemed to be joining against it in the real query.

Aside #2 - You're probably better off with CURRENT_DATE since begin/end seem to be dates, rather than TIMESTAMP 'now' - and in any case you wanted "timestamp with time zone"

OK, I think the root of your problem is your use of null to mean "not ended" or "not started" (whatever 'not started' means). PostgreSQL has the handy timestamptz value "infinity", but only for timestamps and not for dates. I'd probably cheat a little and use an end date of '9999-12-31' or similar to simulate "infinity". Then your test is simply:

  AND (rol.begin <= CURRENT_DATE AND rol.end >= CURRENT_DATE)

That should estimate simply enough.

  Richard Huxton
  Archonet Ltd

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