Re: POC, WIP: OR-clause support for indexes

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
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