database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-06-11 20:28:21 +03:00
Code Activity

Make postgres_fdw's "Relations" output agree with the rest of EXPLAIN.

Browse Source 
The relation aliases shown in the "Relations" line for a foreign scan
didn't always agree with those used in the rest of EXPLAIN's output.
The regression test result changes appearing here provide examples.

It's really impossible for postgres_fdw to duplicate EXPLAIN's alias
assignment logic during postgresGetForeignRelSize(), because of the
de-duplication that EXPLAIN does on a global basis --- and anyway,
trying to duplicate that would be unmaintainable.  Instead, just put
numeric rangetable indexes into the string, and convert those to
table names/aliases in postgresExplainForeignScan, which does have
access to the results of ruleutils.c's alias assignment logic.
Aside from being more reliable, this shifts some work from planning
to EXPLAIN, which is a good tradeoff for performance.  (I also
changed from using StringInfo to using psprintf, which makes the
code slightly simpler and reduces its memory consumption.)

A kluge required by this solution is that we have to reverse-engineer
the rtoffset applied by setrefs.c.  If that logic ever fails
(presumably because the member tables of a join got offset by
different amounts), we'll need some more cooperation with setrefs.c
to keep things straight.  But for now, there's no need for that.

Arguably this is a back-patchable bug fix, but since this is a mostly
cosmetic issue and there have been no field complaints, I'll refrain
for now.

Discussion: https://postgr.es/m/12424.1575168015@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2019-12-02 16:31:03 -05:00
parent 1d468b9ad8
commit 4526951d56
3 changed files with 117 additions and 56 deletions
Download Patch File Download Diff File Expand all files Collapse all files

11
contrib/postgres_fdw/postgres_fdw.h
View File

@ -87,11 +87,14 @@ typedef struct PgFdwRelationInfo
int fetch_size; /* fetch size for this remote table */
/*
* Name of the relation while EXPLAINing ForeignScan. It is used for join
* relations but is set for all relations. For join relation, the name
* indicates which foreign tables are being joined and the join type used.
* Name of the relation, for use while EXPLAINing ForeignScan. It is used
* for join and upper relations but is set for all relations. For a base
* relation, this is really just the RT index as a string; we convert that
* while producing EXPLAIN output. For join and upper relations, the name
* indicates which base foreign tables are included and the join type or
* aggregation type used.
*/
StringInfo relation_name;
char *relation_name;
/* Join information */
RelOptInfo *outerrel;