Re: [HACKERS] [PATCH] Push limit to sort through a subquery

On 18.08.2017 04:33, Robert Haas wrote:

It seems like a somewhat ad-hoc approach; it supposes that we can take any query produced by deparseSelectStmtForRel() and stick a LIMIT clause onto the very end and all will be well.  Maybe that's not a problematic assumption, not sure.  The grammar happens to allow both FOR UPDATE LIMIT n and LIMIT n FOR UPDATE even though only the latter syntax is documented.

--

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

I am not absolutely sure that it is possible to append any query which can be constructed by postgres_fdw for foreign scan with "LIMIT n" clause.
But I also do not know example when it is not possible. As you have mentioned, "FOR UPDATE LIMIT n" is currently recognized by Postgres.

I have inspected deparseSelectStmtForRel function and now I am sure that appending LIMIT to the SQL statement generated by this function will not cause any problems.
It can produce only the following subset of SELECT:

select <target-list> FROM <table-list> [GROUP BY ... [ HAVING ... ]] [ OREDER BY ... ] [ FOR UPDATE ... ];


The only suspicious clause is FOR UPDATE, but I have checked that "FOR UPDATE ... LIMIT n" is  really accepted by Postgres parser.

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Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company