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
CASE
WHEN rol.role_id IS NULL
THEN NULL
WHEN rol."begin" IS NOT NULL and rol."end" IS NOT NULL
THEN
CASE WHEN TIMESTAMP 'now'>=rol."begin" and TIMESTAMP
'now'<=rol."end"
THEN 'Active'
ELSE 'Inactive' END
WHEN rol."begin" IS NOT NULL
THEN
CASE WHEN TIMESTAMP 'now'>=rol."begin"
THEN 'Active'
ELSE 'Inactive' END
WHEN rol."end" IS NOT NULL
THEN
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:
WHERE
...
AND (rol.begin <= CURRENT_DATE AND rol.end >= CURRENT_DATE)
That should estimate simply enough.
--
Richard Huxton
Archonet Ltd
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