Re: Slow query when pg_trgm is in inner lopp

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От Sasa Vilic
Тема Re: Slow query when pg_trgm is in inner lopp
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Msg-id CAOJhpYfAa_z8Jr5otTpfLnP3iPyLZqYfNSLQEPnM4er=HWg61g@mail.gmail.com
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Ответ на Re: Slow query when pg_trgm is in inner lopp  (Jeff Janes <jeff.janes@gmail.com>)
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Hi Jeff,

the way I see it, is it a poor man's implementation of 'full-text' search. I just discussed it with out navdata team and we might be redefine how do we do the search. Regardless of that, I think that issue with Postgres stands.

I tried now to see, how the query would behave if we always had left-anchored pattern and that would allow us to stick to btree indexes.

* Query
------------------------------------
EXPLAIN (ANALYZE, COSTS, VERBOSE, BUFFERS)
SELECT
  r.*
FROM navdata.route r
  INNER JOIN navdata.point op ON r.frompointguid = op.guid
  INNER JOIN navdata.point dp ON r.topointguid = dp.guid
WHERE
  r.routeidentifier LIKE '%'
  AND tsrange(r.startvalid, r.endvalid) @> now() :: TIMESTAMP
  AND (NOT false :: BOOLEAN OR r.source = ANY (ARRAY[] :: VARCHAR []))
  AND op.identifier LIKE 'LOW%'
  AND op.type = ANY (ARRAY['PA'] :: VARCHAR [])
  AND tsrange(op.startvalid, op.endvalid) @> now() :: TIMESTAMP
  AND dp.identifier LIKE '%' :: VARCHAR
  AND dp.type = ANY (ARRAY['PA'] :: VARCHAR [])
  AND tsrange(dp.startvalid, dp.endvalid) @> now() :: TIMESTAMP
ORDER BY r.routeidentifier
LIMIT 1000;

* Analysis
---------------------------------------
Limit  (cost=646.48..646.48 rows=1 width=349) (actual time=1375.359..1376.447 rows=1000 loops=1)
  Output: r.uid, r.routeidentifier, r.frompointguid, r.topointguid, r.sidguid, r.starguid, r.routeinformation, r.routetype, r.startvalid, r.endvalid, r.revisionuid, r.source, r.fufi, r.grounddistance_excl_sidstar, r.from_first, r.dep_airports, r.dst_airports, r.tag, r.expanded_route_string, r.route_geometry
  Buffers: shared hit=79276
  ->  Sort  (cost=646.48..646.48 rows=1 width=349) (actual time=1375.356..1375.785 rows=1000 loops=1)
        Output: r.uid, r.routeidentifier, r.frompointguid, r.topointguid, r.sidguid, r.starguid, r.routeinformation, r.routetype, r.startvalid, r.endvalid, r.revisionuid, r.source, r.fufi, r.grounddistance_excl_sidstar, r.from_first, r.dep_airports, r.dst_airports, r.tag, r.expanded_route_string, r.route_geometry
        Sort Key: r.routeidentifier
        Sort Method: top-N heapsort  Memory: 321kB
        Buffers: shared hit=79276
        ->  Nested Loop  (cost=250.30..646.47 rows=1 width=349) (actual time=202.826..1372.178 rows=2596 loops=1)
              Output: r.uid, r.routeidentifier, r.frompointguid, r.topointguid, r.sidguid, r.starguid, r.routeinformation, r.routetype, r.startvalid, r.endvalid, r.revisionuid, r.source, r.fufi, r.grounddistance_excl_sidstar, r.from_first, r.dep_airports, r.dst_airports, r.tag, r.expanded_route_string, r.route_geometry
              Buffers: shared hit=79276
              ->  Nested Loop  (cost=249.87..621.96 rows=1 width=349) (actual time=202.781..1301.135 rows=2602 loops=1)
                    Output: r.uid, r.routeidentifier, r.frompointguid, r.topointguid, r.sidguid, r.starguid, r.routeinformation, r.routetype, r.startvalid, r.endvalid, r.revisionuid, r.source, r.fufi, r.grounddistance_excl_sidstar, r.from_first, r.dep_airports, r.dst_airports, r.tag, r.expanded_route_string, r.route_geometry
                    Buffers: shared hit=51974
                    ->  Index Scan using idx_point_11 on navdata.point op  (cost=0.43..107.02 rows=2 width=16) (actual time=0.055..0.214 rows=7 loops=1)
                          Output: op.uid, op.guid, op.airportguid, op.identifier, op.icaocode, op.name, op.type, op.coordinates, op.fir, op.navaidfrequency, op.elevation, op.magneticvariance, op.startvalid, op.endvalid, op.revisionuid, op.source, op.leveltype
                          Index Cond: (((op.type)::text = ANY ('{PA}'::text[])) AND ((op.identifier)::text ~>=~ 'LOW'::text) AND ((op.identifier)::text ~<~ 'LOX'::text))
                          Filter: (((op.identifier)::text ~~ 'LOW%'::text) AND (tsrange(op.startvalid, op.endvalid) @> (now())::timestamp without time zone))
                          Rows Removed by Filter: 42
                          Buffers: shared hit=52
                    ->  Bitmap Heap Scan on navdata.route r  (cost=249.44..257.45 rows=2 width=349) (actual time=183.255..185.491 rows=372 loops=7)
                          Output: r.uid, r.routeidentifier, r.frompointguid, r.topointguid, r.sidguid, r.starguid, r.routeinformation, r.routetype, r.startvalid, r.endvalid, r.revisionuid, r.source, r.fufi, r.grounddistance_excl_sidstar, r.from_first, r.dep_airports, r.dst_airports, r.tag, r.expanded_route_string, r.route_geometry
                          Recheck Cond: ((r.frompointguid = op.guid) AND (tsrange(r.startvalid, r.endvalid) @> (now())::timestamp without time zone))
                          Filter: ((r.routeidentifier)::text ~~ '%'::text)
                          Heap Blocks: exact=2140
                          Buffers: shared hit=51922
                          ->  BitmapAnd  (cost=249.44..249.44 rows=2 width=0) (actual time=183.197..183.197 rows=0 loops=7)
                                Buffers: shared hit=49782
                                ->  Bitmap Index Scan on idx_route_02  (cost=0.00..10.96 rows=338 width=0) (actual time=0.162..0.162 rows=884 loops=7)
                                      Index Cond: (r.frompointguid = op.guid)
                                      Buffers: shared hit=47
                                ->  Bitmap Index Scan on idx_route_07  (cost=0.00..237.01 rows=4896 width=0) (actual time=182.858..182.858 rows=579062 loops=7)
                                      Index Cond: (tsrange(r.startvalid, r.endvalid) @> (now())::timestamp without time zone)
                                      Buffers: shared hit=49735
              ->  Index Scan using cidx_point on navdata.point dp  (cost=0.43..24.50 rows=1 width=16) (actual time=0.008..0.025 rows=1 loops=2602)
                    Output: dp.uid, dp.guid, dp.airportguid, dp.identifier, dp.icaocode, dp.name, dp.type, dp.coordinates, dp.fir, dp.navaidfrequency, dp.elevation, dp.magneticvariance, dp.startvalid, dp.endvalid, dp.revisionuid, dp.source, dp.leveltype
                    Index Cond: (dp.guid = r.topointguid)
                    Filter: (((dp.type)::text = ANY ('{PA}'::text[])) AND ((dp.identifier)::text ~~ '%'::text) AND (tsrange(dp.startvalid, dp.endvalid) @> (now())::timestamp without time zone))
                    Rows Removed by Filter: 6
                    Buffers: shared hit=27302
Planning time: 12.202 ms
Execution time: 1376.912 ms

Why think is weird here is this:

                        ->  BitmapAnd  (cost=249.44..249.44 rows=2 width=0) (actual time=183.197..183.197 rows=0 loops=7)
                                Buffers: shared hit=49782
                                ->  Bitmap Index Scan on idx_route_02  (cost=0.00..10.96 rows=338 width=0) (actual time=0.162..0.162 rows=884 loops=7)
                                      Index Cond: (r.frompointguid = op.guid)
                                      Buffers: shared hit=47
                                ->  Bitmap Index Scan on idx_route_07  (cost=0.00..237.01 rows=4896 width=0) (actual time=182.858..182.858 rows=579062 loops=7)
                                      Index Cond: (tsrange(r.startvalid, r.endvalid) @> (now())::timestamp without time zone)
                                      Buffers: shared hit=49735

Why would postgres choose to use second index idx_route_07 at all when row estimate is way higher then on idx_route_02? Wouldn't it be better just to use one index with lower number of estimated rows and then filter?

Thanks
Sasa

On 20 June 2018 at 16:53, Jeff Janes <jeff.janes@gmail.com> wrote:
On Wed, Jun 20, 2018 at 9:21 AM, Sasa Vilic <sasavilic@gmail.com> wrote:
 
Query that we have finds all routes between two set of points. A set is a
dynamically/loosely defined by pattern given by the user input. So for example
if user wants to find all routes between international airports in Austria
toward London Heathrow, he or she would use 'LOW%' as :from_point_identifier
and 'EGLL' as :to_point_identifier. Please keep in mind that is a simple case,
and that user is allowed to define search term any way he/she see it fit,
i.e. '%OW%', 'EG%'.


Letting users do substring searches on airport codes in the middle of a complex query makes no sense.  Do all airports with 'OW' in the middle of them having something in common with each other?  If people can't remember the real airport code of the airport they are using, you should offer a look-up tool which they can use to figure that out **before** hitting the main query.

But taking for granted your weird use case, the most obvious improvement to the PostgreSQL code that I can see is in the executor, not the planner.  There is no reason to recompute the bitmap on idx_point_08 each time through the nested loop, as the outcome of that scan doesn't depend on the outer tuple.  Presumably the reason this happens is that it is being 'BitmapAnd'ed with another bitmap index scan which does depend on the outer tuple, and it is just not smart enough to reuse the stable bitmap while recomputing the parameterized one.

Cheers,

Jeff

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