Re: Planner Row Estimate with Function

Hello

2009/12/29 Michael Fork :
> 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;
>

try
 CREATE FUNCTION parsecardidfromreferencecode(text) RETURNS text AS $_$
SELECT SUBSTRING($1 FROM 3 FOR 13);
 $_$ LANGUAGE sql;

regards
Pavel Stehule

> 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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