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Index usage with functions in where condition

From: Jeremy Palmer <JPalmer(at)linz(dot)govt(dot)nz>
To: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: Index usage with functions in where condition
Date: 2010-07-09 22:36:15
Message-ID: 666FB8D75E95AE42965A0E76A5E5337E065C035C1E@prdlsmmsg01.ad.linz.govt.nz (view raw or flat)
Thread:
Lists: pgsql-performance
I'm having trouble getting the query planner to use indexes. The situation occurs when writing a query that uses functions for defining the parameters for the conditions on the indexed columns. The system I'm running is Windows Server 2003, using version 8.4.2 of PostgreSQL.

This is the following table that I'm running my query against:

CREATE TABLE crs_coordinate
(
  id integer NOT NULL,
  nod_id integer NOT NULL,
  value1 numeric(22,12),
  value2 numeric(22,12),
  CONSTRAINT crs_coordinate_pkey PRIMARY KEY (id)
)
WITH (
  OIDS=FALSE
);

CREATE INDEX coo_value1 ON crs_coordinate USING btree (value1);
CREATE INDEX coo_value2 ON crs_coordinate USING btree (value2);

This table has 23 million rows in it and was analysed just before planning my queries. 

This is the query that does not use the indexes:

SELECT
  coo.nod_id,
  6400000*radians(sqrt((coo.value1 - -41.0618)^2+((coo.value2 - 175.58461)*cos(radians(-41.0618)))^2)) as distance
FROM 
  crs_coordinate coo
WHERE
  coo.value1 between -41.0618-degrees(1200.0/6400000.0) and -41.0618+degrees(1200.0/6400000.0) and
  coo.value2 between 175.58461-degrees(1200.0/6400000.0)/(cos(radians(-41.0618))) and 175.58461+degrees(1200.0/6400000.0)/(cos(radians(-41.0618)));

Seq Scan on crs_coordinate coo  (cost=0.00..1039607.49 rows=592 width=28)
  Filter: (((value1)::double precision >= (-41.0725429586587)::double precision) AND ((value1)::double precision <= (-41.0510570413413)::double precision) AND ((value2)::double precision >= 175.570362072701::double precision) AND ((value2)::double precision <= 175.598857927299::double precision))

However if I pre-evaluated the parameters for the where condition on the value1 and value2 columns, the planner chooses to use the indexes:

SELECT
  coo.nod_id,
  6400000*radians(sqrt((coo.value1 - -41.0618)^2+((coo.value2 - 175.58461)*cos(radians(-41.0618)))^2)) as distance
FROM 
  crs_coordinate coo
WHERE
  coo.value1 BETWEEN -41.07254296 AND -41.05105704 AND
  coo.value2 BETWEEN 175.57036207 AND 175.59885792;

Bitmap Heap Scan on crs_coordinate coo  (cost=5299.61..6705.41 rows=356 width=28)
  Recheck Cond: ((value1 >= (-41.07254296)) AND (value1 <= (-41.05105704)) AND (value2 >= 175.57036207) AND (value2 <= 175.59885792))
  ->  BitmapAnd  (cost=5299.61..5299.61 rows=356 width=0)
        ->  Bitmap Index Scan on coo_value1  (cost=0.00..1401.12 rows=54923 width=0)
              Index Cond: ((value1 >= (-41.07254296)) AND (value1 <= (-41.05105704)))
        ->  Bitmap Index Scan on coo_value2  (cost=0.00..3898.06 rows=153417 width=0)
              Index Cond: ((value2 >= 175.57036207) AND (value2 <= 175.59885792))

So why is the first query not using the indexes on the value1 and value2 columns? I'm assuming that both the COS and RAIDIANS functions are STRICT IMMUTABLE, so logically the evaluation of these functions in the where clause should be inlined. Looking at the query plan this inlining does seem to be happening...

At this stage I have a work around by putting the query into a plpgsql function and using dynamic SQL. But it is still frustrating why the planner seems to be working in a far from optimal fashion. Can anyone shed some light on this for me?

Thanks,
Jeremy

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