On 01.11.2019 18:26, Robert Haas wrote:
> On Fri, Nov 1, 2019 at 11:15 AM Konstantin Knizhnik
> <k.knizhnik@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'scatalog 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.
--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
