I have an index scan on a custom function that is returning a wildly incorrect
row estimate that is throwing off the rest of the query planning. The result
of the function is roughly unique - there are a handful with multiple entries -
but the planner is estimating 227,745 rows. I re-ran ANALYZE on the table and
the results did not change. Any suggestions on how to get more accurate
planner result?
Function definition:
CREATE FUNCTION parsecardidfromreferencecode(text) RETURNS text AS $_$
BEGIN
RETURN SUBSTRING($1 FROM 3 FOR 13);
END;
$_$ LANGUAGE plpgsql IMMUTABLE;
Explain output:
# explain select * from trail.event where type='CREDIT' and
parsecardidfromreferencecode(reference_code) = lpad(CAST('57729970' AS text),
13, '0');
QUERY PLAN
-------------------------------------------------------------------------------------------
Index Scan using idx_event_card_id on event (cost=0.25..468642.89 rows=227745
width=104)
Index Cond: (parsecardidfromreferencecode(reference_code) =
'0000057729970'::text)
Statistics:
# SELECT null_frac, avg_width, n_distinct, most_common_vals, most_common_freqs,
correlation, histogram_bounds FROM pg_stats WHERE tablename =
'idx_event_card_id';
null_frac | avg_width | n_distinct | most_common_vals | most_common_freqs |
correlation |
histogram_bounds
-----------+-----------+------------+------------------+-------------------+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------
0.0935673 | 17 | -1 | | |
0.672617 |
{0000007932138,0000029448430,0000033432203,0000037841145,0000042171261,0000046399858,0000050938838,0000055122354,0003139385377,1220821134582,1261876426760}
Thanks.
Michael
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