Subject: Re: POC, WIP: OR-clause support for indexes
To: Teodor Sigaev <teodor@sigaev.ru>
References: <567ED6CA.2040504@sigaev.ru>
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Cc: David Rowley <david.rowley@2ndquadrant.com>,
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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <0c2fb2ac-332d-6c70-3128-34ba9d13f7e5@2ndquadrant.com>
Date: Sun, 20 Mar 2016 01:44:37 +0100
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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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