Re: SELECT LIMIT 1 VIEW Performance Issue

Dear Mark,

Thank you. That seems like a more manageable alternative if nothing else
works out. It should cover many of the OLTP update transactions. But it
does mean quite a bit of programming changes and adding another index on
all such tables, and it would not cover those cases when we need to get the
latest record before a certain time, for example.

I'm wondering if this performance issue is common enough for other users to
merit a fix in pg, especially as it seems that with MVCC, each of the data
records need to be accessed in addition to scanning the index.

Best regards,
KC.

At 09:40 05/09/24, Mark Kirkwood wrote:
>A small denormalization, where you mark the row with the latest atdate for
>each playerid may get you the performance you want.
>
>e.g: (8.1beta1)
>
>ALTER TABLE player ADD islastatdate boolean;
>
>UPDATE player SET islastatdate = true where (playerid,atdate) IN
>(SELECT playerid, atdate FROM vcurplayer);
>
>CREATE OR REPLACE VIEW vcurplayer AS
>SELECT * FROM player a
>WHERE islastatdate;
>
>CREATE INDEX player_id_lastatdate ON player(playerid, islastatdate)
>WHERE islastatdate;
>
>ANALYZE player;
>
>Generating some test data produced:
>
>EXPLAIN ANALYZE
>SELECT playerid,atdate
>FROM vcurplayer
>WHERE playerid='22220';
>
> QUERY PLAN
>--------------------------------------------------------------------------------------------------------------------------------
> Index Scan using player_id_lastatdate on player a (cost=0.00..4.33
> rows=1 width=13) (actual time=0.142..0.149 rows=1 loops=1)
> Index Cond: ((playerid = '22220'::text) AND (lastatdate = true))
> Filter: lastatdate
> Total runtime: 0.272 ms
>(4 rows)
>
>Whereas with the original view definition:
>
>CREATE OR REPLACE VIEW vcurplayer AS
>SELECT * FROM player a
>WHERE a.atdate =
>( SELECT max(b.atdate) FROM player b
> WHERE a.playerid = b.playerid);
>
>EXPLAIN ANALYZE
>SELECT playerid,atdate
>FROM vcurplayer
>WHERE playerid='22220';
> QUERY PLAN
>-----------------------------------------------------------------------------------------------------------------------------------------------------------------
> Index Scan using player_id_date on player a (cost=0.00..7399.23 rows=11
> width=13) (actual time=121.738..121.745 rows=1 loops=1)
> Index Cond: (playerid = '22220'::text)
> Filter: (atdate = (subplan))
> SubPlan
> -> Result (cost=1.72..1.73 rows=1 width=0) (actual
> time=0.044..0.047 rows=1 loops=2000)
> InitPlan
> -> Limit (cost=0.00..1.72 rows=1 width=4) (actual
> time=0.028..0.031 rows=1 loops=2000)
> -> Index Scan Backward using player_id_date on player
> b (cost=0.00..3787.94 rows=2198 width=4) (actual time=0.019..0.019
> rows=1 loops=2000)
> Index Cond: ($0 = playerid)
> Filter: (atdate IS NOT NULL)
> Total runtime: 121.916 ms
>(11 rows)
>
>Note that my generated data has too many rows for each playerid, but
>the difference in performance should illustrate the idea.
>
>Cheers
>
>Mark