On 01/15/2011 11:52 PM, Randall Smith wrote:
> Hi,
>
> I've created a trigger that checks the uniqueness of two columns in a
> table. Traditionally, one would use a unique constraint, but in my
> case, the size of the unique index would be too large and some
> performance loss is acceptable. However, the trigger performance seems
> to be far below what's indicated by an explain analyze of the query used
> in the trigger.
>
> The unique fields consist of a an indexed int8 (volume_id) and a text
> field (name). The average ratio of volume_id to name is 1 to 10,000.
> The query I'm using to check uniqueness in the trigger is:
A possible work around could be to store an extra field that contains
the md5 hash of the text field and use that field in the index instead
of the text itself. The md5 sum could be calculated by your trigger.
/Fredric
> ...
> IF (SELECT EXISTS (SELECT 1 FROM t1 WHERE
> volume_id = NEW.volume_id AND name = NEW.name)) THEN
> RAISE EXCEPTION '% already exists on volume', NEW.name;
> END IF;
> ...
>
> This trigger is called only BEFORE INSERTS.
>
> In testing, without the trigger, inserts are at 10,000 every 2 seconds.
> With the trigger, they are 10,000 every 40 seconds. The output of
> explain analyze suggests that this shouldn't be taking so long.
>
> EXPLAIN ANALYZE SELECT EXISTS (
> SELECT 1 FROM t1 WHERE volume_id = 300 AND name = 'whodat');
>
> Result (cost=8.35..8.36 rows=1 width=0) (actual time=0.019..0.020
> rows=1 loops=1)
> InitPlan 1 (returns $0)
> -> Index Scan using volume_id_idx on t1 (cost=0.00..8.35 rows=1
> width=0) (actual time=0.016..0.016 rows=0 loops=1)
> Index Cond: (volume_id = 300)
> Filter: (name = 'whodat'::text)
> Total runtime: 0.053 ms
> (6 rows)
>
> 0.053 ms/record / 1,000 ms/sec * 10,000 records = .53 seconds
>
> According to that stat, this lookup should be adding about 0.5 seconds
> to 10,000 records, far from the actual 38 seconds it is adding. I've
> tried to change up the query in the trigger to see if I could get
> different results with not much luck. Any idea what might be taking up
> the extra time or what I can do to troubleshoot?
>
> Thanks.
>
> -Randall
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