On Fri, 4 Apr 2003 11:26:14 -0800, Josh Berkus <josh@agliodbs.com>
wrote:
>For your example, how do the statistics change if you increase the number of
>levels to 15 and put an index on them?
CREATE TABLE baz (event int, level int);
INSERT INTO baz SELECT (100*random()+0.5), (15*random()+0.5);
INSERT INTO baz SELECT (100*random()+0.5), (15*random()+0.5) FROM baz;
...
INSERT INTO baz SELECT (100*random()+0.5), (15*random()+0.5) FROM baz;
ANALYSE baz;
CREATE INDEX baz_e ON baz(event);
CREATE INDEX baz_l ON baz(level);
CREATE INDEX baz_el ON baz(event, level);
CREATE INDEX baz_le ON baz(level, event);
tup cluster case subsel
8K - 1219.90 msec 70605.93 msec (seq scan)
8K - 3087.30 msec (seq scan off)
16K - 3861.87 msec 161902.36 msec (seq scan)
16K - 31498.76 msec (seq scan off)
16K event 2407.72 msec 5773.12 msec
16K level 2298.08 msec 32752.43 msec
16K l, e 2318.60 msec 3184.84 msec
32K - 6571.57 msec 7381.22 msec
32K e, l 4584.97 msec 3429.94 msec
32K l, e 4552.00 msec 64782.59 msec
32K l, e 4552.98 msec 3544.32 msec (baz_l dropped)
64K - 17275.73 msec 26525.24 msec
64K - 17150.16 msec 26195.87 msec (baz_le dropped)
64K - 17286.29 msec 656046.24 msec (baz_el dropped)
64K e, l 9137.88 msec 21809.52 msec
64K e, l 9183.25 msec 6412.97 msec (baz_e dropped)
64K e, l 11690.28 msec 10022.44 msec (baz_el dropped)
64K e, l 11740.54 msec 643046.39 msec (baz_le dropped)
64K l, e 9437.65 msec 133368.20 msec
64K l, e 9119.48 msec 6722.00 msec (baz_l dropped)
64K l, e 9294.68 msec 6663.15 msec (baz_le dropped)
64K l, e 9259.35 msec 639754.27 msec (baz_el dropped)
256K - 59809.69 msec 120755.78 msec
256K - 59809.69 msec 114133.34 msec (baz_le dropped)
256K e, l 38506.41 msec 88531.54 msec
256K e, l 49427.43 msec 43544.03 msec (baz_e dropped)
256K l, e 56821.23 msec 575850.14 msec
256K l, e 57462.78 msec 67911.41 msec (baz_l dropped)
So yes, there are cases where subselect is faster than case, but case
is much more robust regarding correlation and indices.
Servus
Manfred
