Broken scroll cursors with gist index

[martin.f.schaefer]

Hi,

I'm using PostGIS 1.3.1 on PostgreSQL 8.2.4. But the same problem has also been reported for PostGIS 1.3.3 on PostgreSQL 8.3.1.

I have the following table:

CREATE TABLE fog_4752 (

description text, gid integer NOT NULL, item_class text, item_id integer, origin_x double precision, origin_y double precision, origin_z double precision, geometry geometry, CONSTRAINT enforce_dims_geometry CHECK ((ndims(geometry) = 2)), CONSTRAINT enforce_srid_geometry CHECK ((srid(geometry) = 27700)) );

INSERT INTO fog_4752 (description, gid, item_class, item_id, origin_x, origin_y, origin_z, geometry) VALUES ('Polygon', 6, 'Polygon', 6, 270463.5995574299, 660527.33722885954, 0, '0103000020346C0000010000000500000042098568C0E014411917774DA44F26419475BFC6784608411917774DA44F26419475BFC678460841936EDB0B1901224142098568C0E01441936EDB0B1901224142098568C0E014411917774DA44F2641'); INSERT INTO fog_4752 (description, gid, item_class, item_id, origin_x, origin_y, origin_z, geometry) VALUES ('Polygon', 4, 'Polygon', 4, 306782.6950348168, 112627.83974142233, 0, '0103000020346C00000100000005000000288A4FB70C430741E06CA5E47060F240EAA4C6336FD11941E06CA5E47060F240EAA4C6336FD1194130FD41FD044F0241288A4FB70C43074130FD41FD044F0241288A4FB70C430741E06CA5E47060F240'); INSERT INTO fog_4752 (description, gid, item_class, item_id, origin_x, origin_y, origin_z, geometry) VALUES ('Polygon', 5, 'Polygon', 5, 224805.30810014351, 415632.86486705049, 0, '0103000020346C00000100000005000000A0BDB7907EBA04415A4590094F4612417256A12EEB1311415A4590094F4612417256A12EEB1311419892D7F01B3B2041A0BDB7907EBA04419892D7F01B3B2041A0BDB7907EBA04415A4590094F461241'); INSERT INTO fog_4752 (description, gid, item_class, item_id, origin_x, origin_y, origin_z, geometry) VALUES ('Polygon', 1, 'Polygon', 1, 317159.57945692743, 809954.47290725145, 0, '0103000020346C0000010000000500000018E0648798E71641E6B7DC1478FF2A415CE0AA36489F0F41E6B7DC1478FF2A415CE0AA36489F0F41861465CF1170264118E0648798E71641861465CF1170264118E0648798E71641E6B7DC1478FF2A41'); INSERT INTO fog_4752 (description, gid, item_class, item_id, origin_x, origin_y, origin_z, geometry) VALUES ('Polygon', 2, 'Polygon', 2, 457247.5191554199, 527703.21662584448, 0, '0103000020346C00000100000005000000B50BF40E7B642041475B545A4EF4224186DD520906081741475B545A4EF4224186DD5209060817411E9EFD061D821A41B50BF40E7B6420411E9EFD061D821A41B50BF40E7B642041475B545A4EF42241'); INSERT INTO fog_4752 (description, gid, item_class, item_id, origin_x, origin_y, origin_z, geometry) VALUES ('Polygon', 3, 'Polygon', 3, 567242.49402979179, 197718.29200272885, 0, '0103000020346C000001000000050000003FCF4C7C885E23415E698CEE51801041BA452CFB42811E415E698CEE51801041BA452CFB42811E41E075E49D8189FE403FCF4C7C885E2341E075E49D8189FE403FCF4C7C885E23415E698CEE51801041');

ALTER TABLE ONLY fog_4752 ADD CONSTRAINT fog_4752_pkey PRIMARY KEY (gid);

CREATE INDEX fog_4752_geometry_sidx ON fog_4752 USING gist (geometry);

Now I'm running these SQL commands:

SET ENABLE_SEQSCAN = OFF; BEGIN; DECLARE C63 SCROLL CURSOR FOR select * from fog_4752 where (geometry && setsrid('BOX(111697.268 85647.94,655446.012 679205.729)'::box2d,27700) and intersects(geometry,'SRID=27700;POLYGON((655446.011617731 679205.729188659,111697.267899139 679205.729188659,111697.267899139 85647.940243935,655446.011617731 85647.940243935,655446.011617731 679205.729188659))'::geometry)); FETCH ABSOLUTE -1 IN C63; FETCH ABSOLUTE 1 IN C63; FETCH FORWARD 10 IN C63; FETCH ABSOLUTE -1 IN C63; CLOSE C63; END;

The 'FETCH ABSOLUTE -1 IN C63' commands return zero rows, when clearly they should return one row, namely the last row in the cursor, which is not empty.

If I change to ENABLE_SEQSCAN = ON, then the FETCH behave as expected. Also, the combination ENABLE_SEQSCAN = OFF and removing the where clause from the DECLARE statement means that all the FETCH behave as expected too. So somehow the combination of ENABLE_SEQSCAN = OFF *AND* the use of a gist index causes the scroll cursor to break.

I think that the setting of ENABLE_SEQSCAN would be irrelevant if the cursor selected a small enough subset of a large enough table.

To me this seems to be a bug. If it is, can it be fixed?

I have also submitted a bug report against PostgreSQL, as I'm not sure whether this is a PostgreSQL or a PostGIS issue.

Regards,

Martin Schäfer

[mcayland]

Hi Martin,

Thanks for the report. I've narrowed this down further and determined it's a PostgreSQL issue - please see ​http://archives.postgresql.org/pgsql-hackers/2008-09/msg01646.php for gory details and follow-up.

ATB,

Mark.

[mcayland]

Martin,

Please see the follow-up here: ​http://archives.postgresql.org/pgsql-hackers/2008-10/msg00855.php

Note that it looks as if the relevant fixes have been applied to CVS, but only for 8.4 devel and so it is unlikely this well get backpatched :( I would suggest that you download an 8.4 CVS, test, and make sure it works as you would expect.

ATB,

Mark.

[mcayland]

Closing due to the fact this is not a PostGIS bug.

ATB,

Mark.