From cbadfc1f8aa76e5a46a20354d533d6c2171668d3 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Sat, 5 Feb 2022 12:55:44 -0500 Subject: [PATCH] Doc: be clearer that foreign-table partitions need user-added constraints. A very well-informed user might deduce this from what we said already, but I'd bet against it. Lay it out explicitly. While here, rewrite the comment about tuple routing to be more intelligible to an average SQL user. Per bug #17395 from Alexander Lakhin. Back-patch to v11. (The text in this area is different in v10 and I'm not sufficiently excited about this point to adapt the patch.) Discussion: https://postgr.es/m/17395-8c326292078d1a57@postgresql.org --- doc/src/sgml/ddl.sgml | 9 ++++--- doc/src/sgml/ref/create_foreign_table.sgml | 30 +++++++++++++++++----- 2 files changed, 29 insertions(+), 10 deletions(-) diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml index 296f3ff59df..166b7a352df 100644 --- a/doc/src/sgml/ddl.sgml +++ b/doc/src/sgml/ddl.sgml @@ -3791,9 +3791,12 @@ VALUES ('Albany', NULL, NULL, 'NY'); - Partitions can also be foreign tables, although they have some limitations - that normal tables do not; see for - more information. + Partitions can also be foreign + tables, although considerable care is needed because it is then + the user's responsibility that the contents of the foreign table + satisfy the partitioning rule. There are some other restrictions as + well. See for more + information. diff --git a/doc/src/sgml/ref/create_foreign_table.sgml b/doc/src/sgml/ref/create_foreign_table.sgml index 9674670e042..b374d8645db 100644 --- a/doc/src/sgml/ref/create_foreign_table.sgml +++ b/doc/src/sgml/ref/create_foreign_table.sgml @@ -348,11 +348,33 @@ CHECK ( expression ) [ NO INHERIT ] constraints on foreign tables, it does assume that they are correct for purposes of query optimization. If there are rows visible in the foreign table that do not satisfy a declared constraint, queries on - the table might produce incorrect answers. It is the user's + the table might produce errors or incorrect answers. It is the user's responsibility to ensure that the constraint definition matches reality. + + + When a foreign table is used as a partition of a partitioned table, + there is an implicit constraint that its contents must satisfy the + partitioning rule. Again, it is the user's responsibility to ensure + that that is true, which is best done by installing a matching + constraint on the remote server. + + + + + Within a partitioned table containing foreign-table partitions, + an UPDATE that changes the partition key value can + cause a row to be moved from a local partition to a foreign-table + partition, provided the foreign data wrapper supports tuple routing. + However it is not currently possible to move a row from a + foreign-table partition to another partition. + An UPDATE that would require doing that will fail + due to the partitioning constraint, assuming that that is properly + enforced by the remote server. + + Similar considerations apply to generated columns. Stored generated columns are computed on insert or update on the local @@ -362,12 +384,6 @@ CHECK ( expression ) [ NO INHERIT ] generated columns that are consistent with the generation expression. Again, this might result in incorrect query results. - - - While rows can be moved from local partitions to a foreign-table partition - (provided the foreign data wrapper supports tuple routing), they cannot be - moved from a foreign-table partition to another partition. -