Jan-Ivar Mellingen skrev:
> One of our customers discovered that by replacing <>TRUE with =FALSE in
> a query of a table containing 750.000 records reduced the query time
> from about 12 seconds to about 60 milliseconds!
>
> The problematic query looks like this:
> SELECT * FROM AlarmLogg WHERE Logg_avsluttet <> TRUE AND
> Alarm_status='X' ORDER BY ID DESC
>
> If it is changed to this it works as expected:
> SELECT * FROM AlarmLogg WHERE Logg_avsluttet = FALSE AND
> Alarm_status='X' ORDER BY ID DESC
>
> After investigation (on a smaller dataset on my own database) I found
> that the query was resulting in a sequential scan:
>
> "explain analyze SELECT * FROM AlarmLogg WHERE Logg_avsluttet <> TRUE
> AND Alarm_status='X' ORDER BY ID DESC"
> "Sort (cost=49936.96..49936.96 rows=1 width=405) (actual
> time=837.793..837.793 rows=0 loops=1)"
> " Sort Key: id"
> " Sort Method: quicksort Memory: 17kB"
> " -> Seq Scan on alarmlogg (cost=0.00..49936.95 rows=1 width=405)
> (actual time=837.782..837.782 rows=0 loops=1)"
> " Filter: ((logg_avsluttet <> true) AND ((alarm_status)::text =
> 'X'::text))"
> "Total runtime: 837.896 ms"
>
> The modified query gave this result:
> "explain analyze SELECT * FROM AlarmLogg WHERE Logg_avsluttet = FALSE
> AND Alarm_status='X' ORDER BY ID DESC"
> "Sort (cost=8.36..8.37 rows=1 width=405) (actual time=0.032..0.032
> rows=0 loops=1)"
> " Sort Key: id"
> " Sort Method: quicksort Memory: 17kB"
> " -> Index Scan using i_alarmlogg_logg_avsluttet on alarmlogg
> (cost=0.00..8.35 rows=1 width=405) (actual time=0.024..0.024 rows=0
> loops=1)"
> " Index Cond: (logg_avsluttet = false)"
> " Filter: ((NOT logg_avsluttet) AND ((alarm_status)::text =
> 'X'::text))"
> "Total runtime: 0.123 ms"
>
> This is a dramatical difference, but I cannot understand why. In my head
> "<>TRUE" should behave exactly the same as "=FALSE". This looks like a
> bug to me, or am I overlooking something?
>
> This was verified on PostgreSQL 8.3.7, both on Windows Xp and Ubuntu 8.10.
>
> Some relevant details from the table definition:
> CREATE TABLE alarmlogg
> (
> id serial NOT NULL,
> alarm_status character varying(1) DEFAULT ''::character varying,
> logg_avsluttet boolean DEFAULT false,
> ...
> CONSTRAINT alarmlogg_pkey PRIMARY KEY (id)
> )
>
> CREATE INDEX i_alarmlogg_alarm_status
> ON alarmlogg
> USING btree
> (alarm_status);
>
> CREATE INDEX i_alarmlogg_logg_avsluttet
> ON alarmlogg
> USING btree
> (logg_avsluttet);
>
> Regards,
> Jan-Ivar Mellingen
> Securinet AS
>
>
>
Thanks to all who answered my question and helped me discover that I
forgot to consider the NULL values.
After a cup of coffee and a little bit of thinking it became clear that
<>TRUE is not the same as FALSE, and the NULLS are not in the index.
PostgreSQL is a great database, but it does not hurt to think a little
when using it...
Thank You all!
/Jan-Ivar
