Hi Teodor,

Sadly the v4 does not work for me - I do get assertion failures. For example with the example Andreas Karlsson posted in this thread:


CREATE EXTENSION btree_gin;
CREATE TABLE test (a int, b int, c int);
CREATE INDEX ON test USING gin (a, b, c);
INSERT INTO test SELECT i % 7, i % 9, i % 11 FROM generate_series(1, 1000000) i;
EXPLAIN ANALYZE SELECT * FROM test WHERE (a = 3 OR b = 5) AND c = 2;

It seems working, but only until I run ANALYZE on the table. Once I do that, I start getting crashes at this line

    *qualcols = list_concat(*qualcols,
                            list_copy(idx_path->indexqualcols));

in convert_bitmap_path_to_index_clause. Apparently one of the lists is T_List while the other one is T_IntList, so list_concat() errors out.

My guess is that the T_BitmapOrPath branch should do

    oredqualcols = list_concat(oredqualcols, li_qualcols);
    ...
    *qualcols = list_concat(qualcols, oredqualcols);

instead of

    oredqualcols = lappend(oredqualcols, li_qualcols);
    ...
    *qualcols = lappend(*qualcols, oredqualcols);

but once I fixed that I got some other assert failures further down, that I haven't tried to fix.

So the patch seems to be broken, and I suspect this might be related to the broken index condition reported by Andreas (although I don't see that - I either see correct condition or assertion failures).


On 03/17/2016 06:19 PM, Teodor Sigaev wrote:
...

7) I find it rather ugly that the paths are built by converting BitmapOr
paths. Firstly, it means indexes without amgetbitmap can't benefit from
this change. Maybe that's reasonable limitation, though?
I based on following thoughts:
1 code which tries to find OR-index path will be very similar to existing
  generate_or_bitmap code. Obviously, it should not be duplicated.
2 all existsing indexes have amgetbitmap method, only a few don't.
amgetbitmap
  interface is simpler. Anyway, I can add an option for generate_or_bitmap
  to use any index, but, in current state it will just repeat all work.

I agree that the code should not be duplicated, but is this really a good solution. Perhaps a refactoring that'd allow sharing most of the code would be more appropriate.


But more importantly, this design already has a bunch of unintended
consequences. For example, the current code completely ignores
enable_indexscan setting, because it merely copies the costs from the
bitmap path.
>
I'd like to add separate enable_indexorscan

That may be useful, but why shouldn't enable_indexscan=off also disable indexorscan? I would find it rather surprising if after setting enable_indexscan=off I'd still get index scans for OR-clauses.


That's pretty dubious, I guess. So this code probably needs to be made
aware of enable_indexscan - right now it entirely ignores startup_cost
in convert_bitmap_path_to_index_clause(). But of course if there are
multiple IndexPaths, the  enable_indexscan=off will be included multiple
times.

... and it does not address this at all.

I really doubt a costing derived from the bitmap index scan nodes will make much sense - you essentially need to revert unknown parts of the costing to only include building the bitmap once, etc.



regards

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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