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
---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
