From: | Lincoln Yeoh <lylyeoh(at)mecomb(dot)com> |
---|---|
To: | Mike Mascari <mascarm(at)mascari(dot)com> |
Cc: | davidb(at)vectormath(dot)com, pgsql-general(at)postgresql(dot)org |
Subject: | Re: Revisited: Transactions, insert unique. |
Date: | 2000-04-25 07:56:52 |
Message-ID: | 3.0.5.32.20000425155652.008dbb60@pop.mecomb.po.my |
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Thread: | |
Lists: | pgsql-general |
At 10:17 PM 24-04-2000 -0400, Mike Mascari wrote:
>Lincoln Yeoh wrote:
>PostgreSQL implements SELECT...FOR UPDATE to allow for the
>sequence you'be described:
>
>Session 1:
>
>BEGIN;
>SELECT webuser FROM webusers WHERE webuser = 'webuser1';
>
>Session 2:
>
>BEGIN;
>UPDATE webusers SET webuser = 'webuser2' WHERE webuser =
>'webuser1';*
The 6.5.3 running on my site does not block for the case I described which
has an INSERT after the select.
begin;
select from mytable where field=value for update;
if rows=0 {
insert into mytable (field,field2) values (value,value2);
} else {
update mytable set field=value, field2=value2;
}
commit;
Tested on 6.5.3.
Did a select for update in two concurrent transactions, they did not block,
and both inserts went through (if there was a unique, one of the
transactions would have had to be rolled back and redone from scratch).
If the behaviour is different for 7.0, I'm interested!
Cheerio,
Link.
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