From: Florence Cousin <cousinflo@free.fr>
To: pgsql-docs@postgresql.org
Subject: Re: For update clause
Date: Wed, 16 Jun 2010 09:58:15 +0200
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Le mardi 15 juin 2010 22:04:06, Bruce Momjian a =E9crit :
>=20
> Wow, that is a confusing double-negative sentence.  I have updated the
> text to be:
>=20
>     In addition, rows that satisfied the query conditions as of the
>     query snapshot will be locked, although they will not be returned
>     if they were updated after the snapshot and no longer satisfy the
>     query conditions.
>=20
> What it is saying is that SELECT FOR UPDATE will lock all rows that
> match the SELECT query using the current snapshot, but the returned rows
> might be different because the rows were changed after the snapshot was
> taken, and a SELECT FOR UPDATE will return the rows as UPDATE will see
> them, which might not match the SELECT snapshot.  Yeah, it is confusing.

Thank you for the patch and the explanation. It is clear for me now (the ro=
ws=20
returned are those that would be returned by an UPDATE, that is pretty=20
logical).

But I think most of the users will still not understand this, because they =
do=20
not know what a snapshot is, and do not really know how locking works.=20

And I managed to understand thank to the explanation, but I think I could n=
ot=20
understand with the new version of the explanation alone (the fact that row=
s=20
returned are the rows as UPDATE will see them)

Maybe it would be clearer with a longer explanation, or a link to an=20
explanation?