Doc: Update documentation for asynchronous execution.

Add a note of caution on the performance of asynchronous execution by
postgres_fdw.  Follow-up for commit 27e1f1456.

Stephen Frost, a little bit expanded by me.

Discussion: https://postgr.es/m/20210506171224.GV20766%40tamriel.snowman.net

diff --git a/doc/src/sgml/postgres-fdw.sgml b/doc/src/sgml/postgres-fdw.sgml
index 839126c4efe79a6d57d6832da88324be2de62b00..fb87372bde1a261f2f4dd47c7108ede234e0037a 100644 (file)
--- a/doc/src/sgml/postgres-fdw.sgml
+++ b/doc/src/sgml/postgres-fdw.sgml
@@ -401,6 +401,16 @@ OPTIONS (ADD password_required 'false');
        A table-level option overrides a server-level option.
        The default is false.
       
+
+      
+       In order to ensure that the data being returned from a foreign server
+       is consistent, postgres_fdw will only open one
+       connection for a given foreign server and will run all queries against
+       that server sequentially even if there are multiple foreign tables
+       involved, unless those tables are subject to different user mappings.
+       In such a case, it may be more performant to disable this option to
+       eliminate the overhead associated with running queries asynchronously.
+