self join revisited

From: Rikard Pavelic
Subject: self join revisited
Date: ,
Msg-id: 49D3969F.4030701@zg.htnet.hr
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Responses: Re: self join revisited  (Matthew Wakeling)
Re: self join revisited  (Tom Lane)
List: pgsql-performance

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self join revisited  (Rikard Pavelic, )
 Re: self join revisited  (Matthew Wakeling, )
 Re: self join revisited  (Tom Lane, )
  Re: self join revisited  (Rikard Pavelic, )
   Re: self join revisited  (Robert Haas, )

How hard would it be to teach planer to optimize self join?

While this query which demonstrates it is not that common

SELECT count(*)
FROM
    big_table a
    INNER JOIN big_table b ON a.id = b.id;

This type of query (self joining large table) is very common
(at least in our environment because of heavy usage of views).

It would be great if Postgres could rewrite this query

SELECT bt1.id, bt1.total, sq.id, sq.total
FROM
    big_table bt1
    INNER JOIN small_table st1 on st1.big_id = bt1.id
    INNER JOIN
    (
        SELECT bt2.id, st2.total
        FROM
            big_table bt2
            INNER JOIN small_table st2 on st2.big_id = bt2.id
        WHERE
            st2.total > 100
    ) sq ON sq.id = bt1.id
WHERE
    st1.total<200

like this

SELECT bt1.id, bt1.total, bt1.id, st2.total
FROM
    big_table bt1
    INNER JOIN small_table st1 on st1.big_id = bt1.id
    INNER JOIN small_table st2 on st2.big_id = bt1.id AND st2.total > 100
WHERE
    st1.total<200

Regards,
Rikard


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