Re: Another try at reducing repeated detoast work for PostGIS
От | Mark Cave-Ayland |
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Тема | Re: Another try at reducing repeated detoast work for PostGIS |
Дата | |
Msg-id | 4A8A7287.1080208@siriusit.co.uk обсуждение исходный текст |
Ответ на | Another try at reducing repeated detoast work for PostGIS (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: Another try at reducing repeated detoast work for PostGIS
(Tom Lane <tgl@sss.pgh.pa.us>)
|
Список | pgsql-hackers |
Tom Lane wrote: > There was recently another go-round on the postgis-devel list about > the same problem Mark Cave-Ayland complained about last year: > http://archives.postgresql.org/pgsql-hackers/2008-06/msg00384.php > Basically, what is happening is a nestloop join where the inner > indexscan gets a comparison argument from the outer table, and that > argument is toasted. Every single call of an index support function > detoasts the argument again :-(, leading to upwards of 90% of the > runtime being spent in detoasting. > > I made a proposal to fix it > http://archives.postgresql.org/pgsql-hackers/2008-06/msg00709.php > but that failed due to excessive ambition --- the cost/benefit/risk > tradeoffs just weren't good enough. > > Thinking about it again, it seems to me that a much narrower patch > could solve the specific forms of the problem that the PostGIS folk > are seeing. Instead of trying to have a general-purpose method of > preventing repeat de-toasting, we could just prevent it for inner > indexscans by having ExecIndexEvalRuntimeKeys() detoast anything it's > passing to the index AM. The attached patch accomplishes this with > a net addition of about three statements. (It looks bigger, because > I had to move a hunk of code to have the datatype info available when > needed.) Paul Ramsey reports that this fixes the problem for him: > http://postgis.refractions.net/pipermail/postgis-devel/2009-August/006659.html > > The only downside I can see offhand is that it will detoast short-header > values that might not actually need to be detoasted. But we don't have > any very good way to know whether a datatype's index support functions > use PG_DETOAST_DATUM or PG_DETOAST_DATUM_PACKED. In the former case we > do need to detoast short-header values. The extra overhead in this case > amounts to only one palloc and a fairly short memcpy, which should be > pretty negligible in comparison to the other setup costs of an > indexscan, so I'm not very concerned about it. > > Comments? Better ideas? > > regards, tom lane Hi Tom, Thanks for the patch. Fortunately enough I was able to find the dataset from the original report above, and so I've tested your patch against PostgreSQL 8.4. Unfortunately in the original test case, it doesn't seem to give the same performance improvement for me that Paul was seeing :( postgis13=# \d geography Table "public.geography" Column | Type | Modifiers ------------+------------------------+--------------------------------------------------------- gid | integer | not null default nextval('geography_gid_seq'::regclass) type | character varying(1) | id | numeric(20,0) | name | character varying(32) | population | numeric(20,0) | abbreviati | character varying(2) | po_name |character varying(100) | id_geo_sta | numeric(20,0) | the_geom | geometry | centroid | geometry | Indexes: "geography_pkey" PRIMARY KEY, btree (gid) "idx_geography_centroid_z" gist (centroid) Check constraints: "enforce_dims_the_geom" CHECK (ndims(the_geom) = 2) "enforce_geotype_the_geom" CHECK (geometrytype(the_geom)= 'MULTIPOLYGON'::text OR the_geom IS NULL) "enforce_srid_the_geom" CHECK (srid(the_geom) = (-1)) PostgreSQL 8.4 vanilla: postgis13=# explain analyze select count(*) from geography where type='Z' and centroid && (select the_geom from geography where id=69495); QUERY PLAN -------------------------------------------------------------------------------------------------------------------------------------------------- Aggregate (cost=6892.28..6892.29 rows=1 width=0) (actual time=1770.333..1770.334 rows=1 loops=1) InitPlan 1 (returns $0) -> Seq Scan on geography (cost=0.00..6884.00 rows=1width=4432) (actual time=19.529..43.893 rows=1 loops=1) Filter: (id = 69495::numeric) -> Index Scan using idx_geography_centroid_zon geography (cost=0.00..8.28 rows=1 width=0) (actual time=48.897..1730.004 rows=29687 loops=1) Index Cond: (centroid && $0) Filter: ((type)::text = 'Z'::text) Total runtime: 1770.417ms (8 rows) PostgreSQL 8.4 with your patch applied: postgis13=# explain analyze select count(*) from geography where type='Z' and centroid && (select the_geom from geography where id=69495); QUERY PLAN -------------------------------------------------------------------------------------------------------------------------------------------------- Aggregate (cost=6892.28..6892.29 rows=1 width=0) (actual time=1499.414..1499.416 rows=1 loops=1) InitPlan 1 (returns $0) -> Seq Scan on geography (cost=0.00..6884.00 rows=1width=4432) (actual time=18.901..42.648 rows=1 loops=1) Filter: (id = 69495::numeric) -> Index Scan using idx_geography_centroid_zon geography (cost=0.00..8.28 rows=1 width=0) (actual time=43.133..1456.944 rows=29687 loops=1) Index Cond: (centroid && $0) Filter: ((type)::text = 'Z'::text) Total runtime: 1499.498ms (8 rows) PostgreSQL 8.4 with force_2d() wrapper function: postgis13=# explain analyze select count(*) from geography where type='Z' and centroid && (select force_2d(the_geom) from geography where id=69495); QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------------------- Aggregate (cost=6892.28..6892.29 rows=1 width=0) (actual time=285.974..285.975 rows=1 loops=1) InitPlan 1 (returns $0) -> Seq Scan on geography (cost=0.00..6884.00 rows=1width=4432) (actual time=19.278..48.056 rows=1 loops=1) Filter: (id = 69495::numeric) -> Index Scan using idx_geography_centroid_zon geography (cost=0.00..8.28 rows=1 width=0) (actual time=48.375..196.404 rows=29687 loops=1) Index Cond: (centroid && $0) Filter: ((type)::text = 'Z'::text) Total runtime: 286.057 ms So while there is some improvement with your patch, it still comes nowhere close to the speed obtained with wrapping the column in function that deTOASTs the input. Could it be that your patch misses out this particular case? ATB, Mark. -- Mark Cave-Ayland - Senior Technical Architect PostgreSQL - PostGIS Sirius Corporation plc - control through freedom http://www.siriusit.co.uk t: +44 870 608 0063 Sirius Labs: http://www.siriusit.co.uk/labs
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