Re: Multiple-Table-Spanning Joins with ORs in WHERE Clause

-----Original Message-----
From: Igor Neyman 
Sent: Thursday, September 22, 2016 10:33 AM
To: 'Sven R. Kunze' <srkunze@mail.de>; pgsql-performance@postgresql.org
Subject: RE: [PERFORM] Multiple-Table-Spanning Joins with ORs in WHERE Clause


-----Original Message-----
From: pgsql-performance-owner@postgresql.org [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of Sven R.
Kunze
Sent: Thursday, September 22, 2016 9:25 AM
To: pgsql-performance@postgresql.org
Subject: [PERFORM] Multiple-Table-Spanning Joins with ORs in WHERE Clause

Hi pgsql-performance list,


what is the recommended way of doing **multiple-table-spanning joins with ORs in the WHERE-clause**?


Until now, we've used the LEFT OUTER JOIN to filter big_table like so:


SELECT DISTINCT <fields of big_table>
FROM
     "big_table"
     LEFT OUTER JOIN "table_a" ON ("big_table"."id" =
"table_a"."big_table_id")
     LEFT OUTER JOIN "table_b" ON ("big_table"."id" =
"table_b"."big_table_id")
WHERE
     "table_a"."item_id" IN (<handful of items>)
     OR
     "table_b"."item_id" IN (<handful of items>);


However, this results in an awful slow plan (requiring to scan the complete big_table which obviously isn't optimal).
So, we decided (at least for now) to split up the query into two separate ones and merge/de-duplicate the result with
applicationlogic:
 


SELECT <fields of big_table>
FROM
     "big_table" INNER JOIN "table_a" ON ("big_table"."id" =
"table_a"."big_table_id")
WHERE
     "table_a"."item_id" IN (<handful of items>);


SELECT <fields of big_table>
FROM
     "big_table" INNER JOIN "table_b" ON ("big_table"."id" = 
"table_b"."big_table_id")
WHERE
     "table_b"."item_id" IN (<handful of items>);


As you can imagine we would be very glad to solve this issue with a 
single query and without having to re-code existing logic of PostgreSQL. 
But how?


Best,
Sven


PS: if you require EXPLAIN ANALYZE, I can post them as well.

______________________________________________________________________________________________

Another option to try::


SELECT DISTINCT <fields of big_table>
FROM
    "big_table"
    LEFT OUTER JOIN "table_a" ON ("big_table"."id" = "table_a"."big_table_id" AND  "table_a"."item_id" IN (<handful of
items>))
    LEFT OUTER JOIN "table_b" ON ("big_table"."id" = "table_b"."big_table_id" AND "table_b"."item_id" IN (<handful of
items>));

Regards,
Igor Neyman