Re: Ideas about a better API for postgres_fdw remote estimates - Mailing list pgsql-hackers

On 8/31/20 6:19 PM, Ashutosh Bapat wrote:
> On Mon, Aug 31, 2020 at 3:36 PM Andrey V. Lepikhov
> <a.lepikhov@postgrespro.ru> wrote:
>> I agreed that this task needs to implement an API for
>> serialization/deserialization of statistics:
>> pg_load_relation_statistics(json_string text);
>> pg_get_relation_statistics(relname text);
>> We can use a version number for resolving conflicts with different
>> statistics implementations.
>> "Load" function will validate the values[] anyarray while deserializing
>> the input json string to the datatype of the relation column.
> 
> This is a valuable feature. Analysing a foreign table by fetching rows
> from the foreign server isn't very efficient. In fact the current FDW
> API for doing that forges that in-efficiency by requiring the FDW to
> return a sample of rows that will be analysed by the core. That's why
> I see that your patch introduces a new API to get foreign rel stat. I
> don't think there's any point in maintaining these two APIs just for

> ANALYSING table. Instead we should have only one FDW API which will do
> whatever it wants and return statistics that can be understood by the
> core and let core install it in the catalogs. I believe that's doable.
I think the same.
> 
> In case of PostgreSQL it could get the stats available as is from the
> foreign server, convert it into a form that the core understands and
> returns. The patch introduces a new function postgres_fdw_stat() which
> will be available only from version 14 onwards. Can we use
> row_to_json(), which is available in all the supported versions,
> instead?
I started from here. But we need to convert starelid, staop[] stacoll[] 
oids into portable format. Also we need to explicitly specify the type 
of each values[] array. And no one guaranteed that anyarray values[] 
can't contained an array of complex type values, containing oids, that 
can't be correctly converted to database objects on another server...
These considerations required me to add new postgres_fdw_stat() routine 
that can be moved into the core.
> 
> In case of some other foreign server, an FDW will be responsible to
> return statistics in a form that the core will understand. It may
> fetch rows from the foreign server or be a bit smart and fetch the
> statistics and convert.
I don't think I fully understood your idea. Please explain in more 
detail if possible.
> 
> This also means that FDWs will have to deal with the statistics format
> that the core understands and thus will need changes in their code
> with every version in the worst case. But AFAIR, PostgreSQL supports
> different forms of statistics so the problem may not remain that
> severe if FDWs and core agree on some bare minimum format that the
> core supports for long.
I don't think FDW needs to know anything about the internals of 
statistics. It only need to execute query like
"SELECT extract_statistics(namespace.relation);"
and apply the text representation by the function call like this:
store_statistics(const char *stat);
All validation and update pg_statistic operations will be performed into 
the core.
> 
> I think the patch has some other problems like it works only for
> regular tables on foreign server but a foreign table can be pointing
> to any relation like a materialized view, partitioned table or a
> foreign table on the foreign server all of which have statistics
> associated with them.
Ok. It was implemented for discussion, test and as a base of development.

> I didn't look closely but it does not consider
> that the foreign table may not have all the columns from the relation
> on the foreign server or may have different names.
Here we get full statistics from remote server and extract statistics 
only for columns, included into the tuple descriptor of foreign table.

> But I think those
> problems are kind of secondary. We have to agree on the design first. 
+1.
I only want to point out the following. In previous threads statistics 
was converted row-by-row. I want to suggest to serialize all statistics 
tuples for the relation into single json string. On apply phase we can 
filter unneeded attributes.

-- 
regards,
Andrey Lepikhov
Postgres Professional