Re: Problem with slow query with WHERE conditions with OR clause on primary keys - Mailing list pgsql-performance

Krzysztof Olszewski wrote
> Hi,
>
> my sql is very simple,
> returns one row,
> where conditions are assigned to primary keys
>
>
> */select g.gd_index, gd.full_name/**/
> /**/from gd g join gd_data gd on (g.id_gd = gd.id_gd)/**/
> /**/where gd.id_gd_data = 1111 OR g.id_gd = 1111;/*
>
>
> but generates "crazy" plan with Merge Join on big amount of rows (both
> tables contains 500000 rows)
> because Index scans ignore conditions, conditions are processed after
> index sacans on Merge Join
>
> */Merge Join  (cost=0.00..46399.80 rows=2 width=115) (actual
> time=3.881..644.409 rows=1 loops=1)/**/
> /**/   Merge Cond: (g.id_gd = gd.id_gd)/**/
> /**/   Join Filter: ((gd.id_gd_data = 1111) OR (g.id_gd = 1111))/**/
> /**/   ->  Index Scan using pk_gd on gd g  (cost=0.00..14117.79
> rows=500001 width=40) (actual time=0.019..146.521 rows=500001 loops=1)/**/
> /**/   ->  Index Scan using fki_gd on gd_data gd  (cost=0.00..22282.04
> rows=500001 width=99) (actual time=0.016..157.384 rows=500001 loops=1)/**/
> /**/Total runtime: 644.460 ms/*
>
>
> model is very simple
>
>
> /CREATE TABLE gd (//
> //   id_gd bigint NOT NULL,//
> //   gd_index character varying(60) NOT NULL,//
> //   notes text,//
> //   notes_exists integer NOT NULL DEFAULT 0,//
> //   CONSTRAINT pk_gd PRIMARY KEY (id_gd )//
> //)//
> //
> //
> //CREATE TABLE gd_data (//
> //   id_gd_data bigint NOT NULL,//
> //   id_gd bigint NOT NULL,//
> //   short_name character varying(120) NOT NULL,//
> //   full_name character varying(512) NOT NULL,//
> //   notes text,//
> //   notes_exists integer NOT NULL DEFAULT 0,//
> //   CONSTRAINT pk_gd_data PRIMARY KEY (id_gd_data ),//
> //   CONSTRAINT fk_gd FOREIGN KEY (id_gd)//
> //       REFERENCES gd (id_gd) MATCH SIMPLE//
> //       ON UPDATE NO ACTION ON DELETE NO ACTION//
> //)//
> //
> //CREATE INDEX fki_gd//
> //   ON gd_data//
> //   USING btree//
> //   (id_gd );//
> /
>
>
> my configuration from (select * from pg_settings):
>
> "server_version";"9.1.10"
>
> Thank you for your help.
>
>
> Kris Olszewski

It cannot do any better since it cannot pre-filter either table using the
where condition without risking removing rows that would meet the other
table's condition post-join.

The query you are executing makes no sense to me: I don't understand why you
would ever filter on gd.id_gd_data given the model you are showing.

I believe your understanding of your model - or the model itself - is flawed
but as you have only provided code it is impossible to pinpoint where
exactly the disconnect resides.  You can either fix the model or the query -
the later by implementing sub-selects with where clauses manually - which
then encodes an assumption about your data that the current query cannot
make.

Your model implies that a single gd record can have multiple gd_data records
associated with it.

David J.





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