From: | Timokhin Maxim <ncx2(at)yandex(dot)com> |
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To: | Sergei Kornilov <sk(at)zsrv(dot)org>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, "pgsql-performance(at)lists(dot)postgresql(dot)org" <pgsql-performance(at)lists(dot)postgresql(dot)org> |
Subject: | Re: Updating a large table |
Date: | 2018-01-09 16:31:17 |
Message-ID: | 585321515515477@web34g.yandex.ru |
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Lists: | pgsql-performance |
Hello, Sergey!
09.01.2018, 15:53, "Sergei Kornilov" <sk(at)zsrv(dot)org>:
> Hello
>
>> 1. ALTER TABLE clusters ADD COLUMN "is_paid" BOOLEAN DEFAULT FALSE;
>
> this is wrong. To avoid large table lock you need DEFAULT NULL:
> ALTER TABLE clusters ADD COLUMN "is_paid" BOOLEAN DEFAULT NULL;
> Default null changes only system catalog, default with any non-null value will rewrite all rows. After adding column you can set default value - it applied only for future inserts:
> ALTER TABLE clusters ALTER COLUMN "is_paid" SET DEFAULT FALSE;
>
> And then you can update all old rows in table by small chunks. Finally, when here is no NULL values you can set not null:
What you wrote are exactly I'm doing. Moreover, I'm checking current metrics to avoid previously problems.
> ALTER TABLE clusters ALTER COLUMN "is_paid" SET NOT NULL;
> But unfortunately this locks table for some time - smaller what rewrite time, but time of full seqscan. I hope my patch [1] will be merged and not null can be set in future by temporary adding check constraint (not valid, then validate) - which not require large table lock
Hope your commit will be merged. It will be realy useful.
>
> [1] https://www.postgresql.org/message-id/flat/81911511895540(at)web58j(dot)yandex(dot)ru#81911511895540@web58j.yandex.ru
>
> Regards, Sergei
--
Timokhin 'maf' Maxim
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