Re: Improvement of procArray.xmin for VACUUM

Bruce Momjian wrote:

> The attached patch updates procArray.xmin in this manner. Here is an
> example of how the patch improves dead row reuse:

I don't think this really works. Consider what happens if I change
session 2 this way:

> Session #:
> 1 2 3
>
> CREATE TABLE test(x int);
> INSERT INTO test VALUES (1);
> BEGIN;
> DELETE FROM test;
> BEGIN;
DECLARE foo CURSOR FOR
SELECT * FROM test;
> SELECT 1;
> COMMIT;
> VACUUM VERBOSE test;
> (row not reused)
> SELECT 1;
FETCH * FROM foo;
>
> (At this point #2 doesn't see
> the test row anymore. Patch
> updates procArray.xmin.)
>
> VACUUM VERBOSE test;
> (row reused with patch)
> COMMIT;
> VACUUM VERBOSE test;
> (normal row reuse)

--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.