On 06/02/2011 11:31 AM, Shaun Thomas wrote:
On 06/02/2011 11:15 AM, Kevin Grittner wrote:
They all gave the same result, of course, and they all used a seq
scan..
And they all will. I created a test table with a bunch of
generate_series and emulated 200 unique matches of column1 and
column2, on a table with a mere 1-million rows (5000 for each of
column3). And no matter what index combination I used, it always did a
sequence scan... even when I indexed every column and indexed column3
descending.
But here's the thing. I turned off sequence scans to force index
scans, and it got 2-3x slower. But is that really surprising? Without
a proper where exclusion, it has to probe every occurrence... also
known as a loose index scan, which PostgreSQL doesn't have (yet).
And... this is horrifying, but:
WITH RECURSIVE t1 AS (
SELECT min(f.tds_cx_ind) AS tds_cx_ind
FROM max_xtrv_st_t f
UNION ALL
SELECT (SELECT min(tds_cx_ind)
FROM max_xtrv_st_t f
WHERE f.tds_cx_ind > t1.tds_cx_ind)
FROM t1
WHERE t1.tds_cx_ind IS NOT NULL
), t2 AS (
SELECT min(f.cxs_ind_2) AS cxs_ind_2
FROM max_xtrv_st_t f
UNION ALL
SELECT (SELECT min(cxs_ind_2)
FROM max_xtrv_st_t f
WHERE f.cxs_ind_2 > t2.cxs_ind_2)
FROM t2
WHERE t2.cxs_ind_2 IS NOT NULL
)
SELECT t1.tds_cx_ind, t2.cxs_ind_2 FROM t1, t2
WHERE t1.tds_cx_ind IS NOT NULL
AND t2.cxs_ind_2 IS NOT NULL;
It works on my test, but might not be what OP wants. It's a cross
product of the two unique column sets, and it's possible it represents
combinations that don't exist. But I suppose a late EXISTS pass could
solve that problem.
I assume there's an easier way to do that. In either case, when is PG
getting loose index scans? ;)
Thanks everyone for the feedback. I'll attempt the suggestions from
today as soon as I can and let you know where we end up.
--
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Kevin Kempter - Constent State
A PostgreSQL Professional Services Company
www.consistentstate.com
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