From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | david(dot)sahagian(at)emc(dot)com |
Cc: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: successive select statements |
Date: | 2011-11-22 02:17:54 |
Message-ID: | 29477.1321928274@sss.pgh.pa.us |
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Lists: | pgsql-general |
<david(dot)sahagian(at)emc(dot)com> writes:
> In postgresql.org/docs/9.1/static/transaction-iso.html I read
> 13.2.1. Read Committed Isolation Level
> . . . two successive SELECT commands can see different data, even though they are within a single transaction . . .
> Please consider this code being executed by postgres:
> = = = = = = = = = =
> select some_int from tableX
> union all
> select another_int from tableY
> ;
> = = = = = = = = = =
> Are these two select statements considered "successive" when isolation == Read Committed ?
No, the UNION is a single command. The fact that the word "SELECT"
occurs twice within it is a SQL syntactic artifact --- it doesn't
convert it into two commands.
regards, tom lane
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