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Re: [HACKERS] [PATCH] Push limit to sort through a subquery - Mailing list pgsql-hackers

From Douglas Doole
Subject Re: [HACKERS] [PATCH] Push limit to sort through a subquery
Date
Msg-id CADE5jY+VaJgOxAhRXiUuj8BSO4FMj_Hrkba5dbXApvCokEi_-Q@mail.gmail.com
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In response to Re: [HACKERS] [PATCH] Push limit to sort through a subquery  (Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>)
Responses Re: [HACKERS] [PATCH] Push limit to sort through a subquery  (Konstantin Knizhnik <k.knizhnik@postgrespro.ru>)
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If you add this to the commitfest app, more people might look at it when the next commitfest opens.


Also, it might help if you can provide a query/ies with numbers where this optimization shows improvement.

I can't provide the real queries where we encountered the problem because they are internal. However I showed a simplified version of the queries in my first post.

On our queries, the change made quite a difference - execution time dropped from 31.4 seconds to 7.2 seconds. Explain analyze also shows that memory use dropped significantly and we didn't have to spill the sort to disk

From:

-> Sort (cost=989.95..1013.27 rows=9326 width=30) (node_startup_time/loop=31328.891, node_total_time/loop: 31329.756 rows=2001 loops=1) Buffers: temp read=772 written=11201 lsm_bufmgr hits=3392 Sort Key: *** Sort Method: external merge Sort Space Used: 89592 Sort Space Type: Disk

To:

-> Sort (cost=989.95..1013.27 rows=9326 width=30) (node_startup_time/loop=7123.275, node_total_time/loop: 7123.504 rows=2001 loops=1) Buffers: lsm_bufmgr hits=3387 Sort Key: *** Sort Method: top-N heapsort Sort Space Used: 3256 Sort Space Type: Memory

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