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From: Tom Lane <[email protected]>
To: Streamsoft - Mirek Szajowski <[email protected]>
Cc: [email protected]
Subject: Re: Locking concurrency: select for update vs update
Date: Tue, 07 Jun 2016 09:24:10 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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Streamsoft - Mirek Szajowski <[email protected]> writes:
> Why I can't execute 'select for update' but I can update?

In recent PG versions, the lock held due to having inserted an FK
dependent row effectively only locks the key fields of the parent row.
UPDATE can tell whether you're trying to change the row's key fields,
and it will proceed if you aren't.  SELECT FOR UPDATE has to lock the
whole row (since it must assume you might be intending to change any
fields of the row); so it blocks until the FK lock goes away.

			regards, tom lane


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