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Re: pgsql_fdw in contrib

From: Kohei KaiGai <kaigai(at)kaigai(dot)gr(dot)jp>
To: Shigeru HANADA <shigeru(dot)hanada(at)gmail(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: pgsql_fdw in contrib
Date: 2012-07-03 07:54:20
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers

Regarding to the issue around sub-transaction abort, an ideal
solution might be execution of SAVEPOINT command on remote
side synchronously. It allows to rollback the active transaction
into the savepoint on the remote server when local one get
However, I'm not inclined to merge anything ideal within a single
patch, and I'd like to recommend to keep your idea simple then
revise the functionality on the upcoming three commit fest.

How about your opinion? The author is you.

2012/6/26 Kohei KaiGai <kaigai(at)kaigai(dot)gr(dot)jp>:
> Harada-san,
> I checked your patch, and had an impression that includes many
> improvements from the previous revision that I looked at the last
> commit fest.
> However, I noticed several points to be revised, or investigated.
> * It seems to me expected results of the regression test is not
>   attached, even though test cases were included. Please add it.
> * cleanup_connection() does not close the connection in case
>   when this callback was invoked due to an error under sub-
>   transaction. It probably makes problematic behavior.
>   See the following steps to reproduce the matter:
> postgres=# BEGIN;
> postgres=# SELECT * FROM ft3;
>  a |  b
> ---+-----
>  1 | aaa
>  2 | bbb
>  3 | ccc
>  4 | ddd
>  5 | eee
>  6 | fff
> (6 rows)
> postgres=# SAVEPOINT sv_01;
> postgres=# SELECT * FROM ft3 WHERE 1 / (a - 4) > 0;  -- should be
> error on remote transaction
> ERROR:  could not execute remote query
> DETAIL:  ERROR:  division by zero
> HINT:  SELECT a, b FROM public.t1 WHERE (((1 OPERATOR(pg_catalog./) (a
> OPERATOR(pg_catalog.-) 4)) OPERATOR(pg_catalog.>) 0))
> postgres=# ROLLBACK TO SAVEPOINT sv_01;
> postgres=# SELECT * FROM ft3;
> ERROR:  could not execute EXPLAIN for cost estimation
> DETAIL:  ERROR:  current transaction is aborted, commands ignored
> until end of transaction block
> HINT:  SELECT a, b FROM public.t1
>   Once we got an error under the remote transaction, it eventually raises
>   an error on local side, then cleanup_connection() should be invoked.
>   But it ignores the error due to sub-transaction, thus, the remote transaction
>   being already aborted is kept.
>   I'd like to suggest two remedy.
>   1. connections are closed, even if errors on sub-transaction.
>   2. close the connection if PQexecParams() returns an error,
>       on execute_query() prior to raise a local error.
>   * Regarding to deparseSimpleSql(), it pulls attributes being referenced
>     from baserestrictinfo and reltargetlist using pull_var_clause().
>     Is it unavailable to use local_conds instead of baserestrictinfo?
>     We can optimize references to the variable being consumed at the
>     remote side only. All we need to transfer is variables referenced
>     in targetlist and local-clauses.
>     In addition, is pull_var_clause() reasonable to list up all the attribute
>     referenced at the both expression tree? It seems to be pull_varattnos()
>     is more useful API in this situation.
>   * Regarding to deparseRelation(), it scans simple_rte_array to fetch
>     RangeTblEntry with relation-id of the target foreign table.
>     It does not match correct entry if same foreign table is appeared in
>     this list twice or more, like a case of self-join. I'd like to recommend
>     to use simple_rte_array[baserel->relid] instead.
>     In addition, it checks whether relkind is RELKIND_FOREIGN_TABLE,
>     or not. It seems to me this check should be Assert(), if routines of
>     pgsql_fdw is called towards regular relations.
>   * Regarding to deparseVar(), is it unnecessary to check relkind of
>     the relation being referenced by Var node, isn't it?
>   * Regarding to deparseBoolExpr(), compiler raised a warning
>     because op can be used without initialized.
>   * Even though it is harmless, sortConditions() is a misleading function
>     name. How about categolizeConditions() or screeningConditions()?
> Thanks for your great jobs.
> 2012/6/14 Shigeru HANADA <shigeru(dot)hanada(at)gmail(dot)com>:
>> I'd like to propose pgsql_fdw, FDW for PostgreSQL, as a contrib module
>> in core, again.  This patch is basically rebased version of the patches
>> proposed in 9.2 development cycle, and contains some additional changes
>> such as concern about volatile variables for PG_CATCH blocks.  In
>> addition, I combined old three patches (pgsql_fdw core functionality,
>> push-down support, and analyze support) into one patch for ease of
>> review. (I think these features are necessary for pgsql_fdw use.)
>> After the development for 9.2 cycle, pgsql_fdw can gather statistics
>> from remote side by providing sampling function to analyze module in
>> backend core, use them to estimate selectivity of WHERE clauses which
>> will be pushed-down, and retrieve query results from remote side with
>> custom row processor which prevents memory exhaustion from huge result set.
>> It would be possible to add some more features, such as ORDER BY
>> push-down with index information support, without changing existing
>> APIs, but at first add relatively simple pgsql_fdw and enhance it seems
>> better.  In addition, once pgsql_fdw has been merged, it would help
>> implementing proof-of-concept of SQL/MED-related features.
>> Regards,
>> --
>> Shigeru HANADA
>> --
>> Sent via pgsql-hackers mailing list (pgsql-hackers(at)postgresql(dot)org)
>> To make changes to your subscription:
> --
> KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>

KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>

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