On 22.08.2017 17:27, Konstantin Knizhnik wrote:
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.
--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
