On 02.11.2019 10:19, Julien Rouhaud wrote:
On Sat, Nov 2, 2019 at 6:31 AM Pavel Stehule <pavel.steh...@gmail.com> wrote:
pá 1. 11. 2019 v 17:09 odesílatel Konstantin Knizhnik
<k.knizh...@postgrespro.ru> napsal:
On 01.11.2019 18:26, Robert Haas wrote:
On Fri, Nov 1, 2019 at 11:15 AM Konstantin Knizhnik
<k.knizh...@postgrespro.ru> wrote:
It seems to me that I have found quite elegant solution for per-backend
statistic for GTT: I just inserting it in backend's catalog cache, but not in
pg_statistic table itself.
To do it I have to add InsertSysCache/InsertCatCache functions which insert
pinned entry in the correspondent cache.
I wonder if there are some pitfalls of such approach?
That sounds pretty hackish. You'd have to be very careful, for
example, that if the tables were dropped or re-analyzed, all of the
old entries got removed --
I have checked it:
- when table is reanalyzed, then cache entries are replaced.
- when table is dropped, then cache entries are removed.
and then it would still fail if any code
tried to access the statistics directly from the table, rather than
via the caches. My assumption is that the statistics ought to be
stored in some backend-private data structure designed for that
purpose, and that the code that needs the data should be taught to
look for it there when the table is a GTT.
Yes, if you do "select * from pg_statistic" then you will not see
statistic for GTT in this case.
But I do not think that it is so critical. I do not believe that anybody
is trying to manually interpret values in this table.
And optimizer is retrieving statistic through sys-cache mechanism and so
is able to build correct plan in this case.
Years ago, when I though about it, I wrote patch with similar design. It's
working, but surely it's ugly.
I have another idea. Can be pg_statistics view instead a table?
Some like
SELECT * FROM pg_catalog.pg_statistics_rel
UNION ALL
SELECT * FROM pg_catalog.pg_statistics_gtt();
Internally - when stat cache is filled, then there can be used
pg_statistics_rel and pg_statistics_gtt() directly. What I remember, there was
not possibility to work with queries, only with just relations.
It'd be a loss if you lose the ability to see the statistics, as there
are valid use cases where you need to see the stats, eg. understanding
why you don't get the plan you wanted. There's also at least one
extension [1] that allows you to backup and use restored statistics,
so there are definitely people interested in it.
[1]: https://github.com/ossc-db/pg_dbms_stats
It seems to have completely no sense to backup and restore statistic for
temporary tables which life time is limited to life time of backend,
doesn't it?
