Re: Reviewing freeze map code

On Mon, Jun 6, 2016 at 2:35 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> > Why would they have to write the complex query? Wouldn't they just
>> > need to run that we wrote for them?
>
> Then write that query. Verify that that query performs halfway
> reasonably fast. Document that it should be run against databases after
> subjecting them to tests. That'd address my concern as well.

Here is a first attempt at such a query. It requires that the
pageinspect and pg_visibility extensions be installed.

SELECT c.oid, v.blkno, array_agg(hpi.lp) AS affect_lps FROM pg_class
c, LATERAL ROWS FROM (pg_visibility(c.oid)) v, LATERAL ROWS FROM
(heap_page_items(get_raw_page(c.oid::regclass::text, blkno::int4)))
hpi WHERE c.relkind IN ('r', 't', 'm') AND v.all_frozen AND
(((hpi.t_infomask & 768) != 768 AND hpi.t_xmin NOT IN (1, 2)) OR
(hpi.t_infomask & 2048) != 2048) GROUP BY 1, 2 ORDER BY 1, 2;

I am not sure this is 100% correct, especially the XMAX-checking part:
is HEAP_XMAX_INVALID guaranteed to be set on a fully-frozen tuple? Is
the method of constructing the first argument to get_raw_page() going
to be robust in all cases?

I'm not sure what the performance will be on a large table, either.
That will have to be checked. And I obviously have not done extensive
stress runs yet. But maybe it's a start. Comments?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company