postgres_fdw: Allow partitions specified in LIMIT TO to be imported.

Commit f49bcd4 disallowed postgres_fdw to import table partitions.
Because all data can be accessed through the partitioned table which
is the root of the partitioning hierarchy, importing only partitioned
table should allow access to all the data without creating extra objects.
This is a reasonable default when importing a whole schema. But there
may be the case where users want to explicitly import one of
a partitioned tables' partitions.

For that use case, this commit allows postgres_fdw to import tables or
foreign tables which are partitions of some other table only when they
are explicitly specified in LIMIT TO clause.  It doesn't change
the behavior that any partitions not specified in LIMIT TO are
automatically excluded in IMPORT FOREIGN SCHEMA command.

Author: Matthias van de Meent
Reviewed-by: Bernd Helmle, Amit Langote, Michael Paquier, Fujii Masao
Discussion: https://postgr.es/m/CAEze2Whwg4i=mzApMe+PXxCEfgoZmHGqdqQFW7J4bmj_5p6t1A@mail.gmail.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -8228,6 +8228,8 @@ ALTER TABLE import_source."x 5" DROP COLUMN c1;
 CREATE TABLE import_source.t4 (c1 int) PARTITION BY RANGE (c1);
 CREATE TABLE import_source.t4_part PARTITION OF import_source.t4
   FOR VALUES FROM (1) TO (100);
+CREATE TABLE import_source.t4_part2 PARTITION OF import_source.t4
+  FOR VALUES FROM (100) TO (200);
 CREATE SCHEMA import_dest1;
 IMPORT FOREIGN SCHEMA import_source FROM SERVER loopback INTO import_dest1;
 \det+ import_dest1.*
@@ -8419,27 +8421,29 @@ FDW options: (schema_name 'import_source', table_name 'x 5')
 
 -- Check LIMIT TO and EXCEPT
 CREATE SCHEMA import_dest4;
-IMPORT FOREIGN SCHEMA import_source LIMIT TO (t1, nonesuch)
+IMPORT FOREIGN SCHEMA import_source LIMIT TO (t1, nonesuch, t4_part)
   FROM SERVER loopback INTO import_dest4;
 \det+ import_dest4.*
-                                     List of foreign tables
-    Schema    | Table |  Server  |                  FDW options                   | Description 
---------------+-------+----------+------------------------------------------------+-------------
- import_dest4 | t1    | loopback | (schema_name 'import_source', table_name 't1') | 
-(1 row)
+                                        List of foreign tables
+    Schema    |  Table  |  Server  |                     FDW options                     | Description 
+--------------+---------+----------+-----------------------------------------------------+-------------
+ import_dest4 | t1      | loopback | (schema_name 'import_source', table_name 't1')      | 
+ import_dest4 | t4_part | loopback | (schema_name 'import_source', table_name 't4_part') | 
+(2 rows)
 
-IMPORT FOREIGN SCHEMA import_source EXCEPT (t1, "x 4", nonesuch)
+IMPORT FOREIGN SCHEMA import_source EXCEPT (t1, "x 4", nonesuch, t4_part)
   FROM SERVER loopback INTO import_dest4;
 \det+ import_dest4.*
-                                     List of foreign tables
-    Schema    | Table |  Server  |                   FDW options                   | Description 
---------------+-------+----------+-------------------------------------------------+-------------
- import_dest4 | t1    | loopback | (schema_name 'import_source', table_name 't1')  | 
- import_dest4 | t2    | loopback | (schema_name 'import_source', table_name 't2')  | 
- import_dest4 | t3    | loopback | (schema_name 'import_source', table_name 't3')  | 
- import_dest4 | t4    | loopback | (schema_name 'import_source', table_name 't4')  | 
- import_dest4 | x 5   | loopback | (schema_name 'import_source', table_name 'x 5') | 
-(5 rows)
+                                        List of foreign tables
+    Schema    |  Table  |  Server  |                     FDW options                     | Description 
+--------------+---------+----------+-----------------------------------------------------+-------------
+ import_dest4 | t1      | loopback | (schema_name 'import_source', table_name 't1')      | 
+ import_dest4 | t2      | loopback | (schema_name 'import_source', table_name 't2')      | 
+ import_dest4 | t3      | loopback | (schema_name 'import_source', table_name 't3')      | 
+ import_dest4 | t4      | loopback | (schema_name 'import_source', table_name 't4')      | 
+ import_dest4 | t4_part | loopback | (schema_name 'import_source', table_name 't4_part') | 
+ import_dest4 | x 5     | loopback | (schema_name 'import_source', table_name 'x 5')     | 
+(6 rows)
 
 -- Assorted error cases
 IMPORT FOREIGN SCHEMA import_source FROM SERVER loopback INTO import_dest4;

contrib/postgres_fdw/postgres_fdw.c

@@ -5095,9 +5095,11 @@ postgresImportForeignSchema(ImportForeignSchemaStmt *stmt, Oid serverOid)
 		 * should save a few cycles to not process excluded tables in the
 		 * first place.)
 		 *
-		 * Ignore table data for partitions and only include the definitions
-		 * of the root partitioned tables to allow access to the complete
-		 * remote data set locally in the schema imported.
+		 * Import table data for partitions only when they are explicitly
+		 * specified in LIMIT TO clause. Otherwise ignore them and only
+		 * include the definitions of the root partitioned tables to allow
+		 * access to the complete remote data set locally in the schema
+		 * imported.
 		 *
 		 * Note: because we run the connection with search_path restricted to
 		 * pg_catalog, the format_type() and pg_get_expr() outputs will always
@@ -5153,7 +5155,8 @@ postgresImportForeignSchema(ImportForeignSchemaStmt *stmt, Oid serverOid)
 		deparseStringLiteral(&buf, stmt->remote_schema);
 
 		/* Partitions are supported since Postgres 10 */
-		if (PQserverVersion(conn) >= 100000)
+		if (PQserverVersion(conn) >= 100000 &&
+			stmt->list_type != FDW_IMPORT_SCHEMA_LIMIT_TO)
 			appendStringInfoString(&buf, " AND NOT c.relispartition ");
 
 		/* Apply restrictions for LIMIT TO and EXCEPT */

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -2366,6 +2366,8 @@ ALTER TABLE import_source."x 5" DROP COLUMN c1;
 CREATE TABLE import_source.t4 (c1 int) PARTITION BY RANGE (c1);
 CREATE TABLE import_source.t4_part PARTITION OF import_source.t4
   FOR VALUES FROM (1) TO (100);
+CREATE TABLE import_source.t4_part2 PARTITION OF import_source.t4
+  FOR VALUES FROM (100) TO (200);
 
 CREATE SCHEMA import_dest1;
 IMPORT FOREIGN SCHEMA import_source FROM SERVER loopback INTO import_dest1;
@@ -2386,10 +2388,10 @@ IMPORT FOREIGN SCHEMA import_source FROM SERVER loopback INTO import_dest3
 
 -- Check LIMIT TO and EXCEPT
 CREATE SCHEMA import_dest4;
-IMPORT FOREIGN SCHEMA import_source LIMIT TO (t1, nonesuch)
+IMPORT FOREIGN SCHEMA import_source LIMIT TO (t1, nonesuch, t4_part)
   FROM SERVER loopback INTO import_dest4;
 \det+ import_dest4.*
-IMPORT FOREIGN SCHEMA import_source EXCEPT (t1, "x 4", nonesuch)
+IMPORT FOREIGN SCHEMA import_source EXCEPT (t1, "x 4", nonesuch, t4_part)
   FROM SERVER loopback INTO import_dest4;
 \det+ import_dest4.*
 

doc/src/sgml/postgres-fdw.sgml

@@ -510,10 +510,12 @@ OPTIONS (ADD password_required 'false');
 
    <para>
     Tables or foreign tables which are partitions of some other table are
-    automatically excluded.  Partitioned tables are imported, unless they
-    are a partition of some other table.  Since all data can be accessed
-    through the partitioned table which is the root of the partitioning
-    hierarchy, this approach should allow access to all the data without
+    imported only when they are explicitly specified in
+    <literal>LIMIT TO</literal> clause.  Otherwise they are automatically
+    excluded from <xref linkend="sql-importforeignschema"/>.
+    Since all data can be accessed through the partitioned table
+    which is the root of the partitioning hierarchy, importing only
+    partitioned tables should allow access to all the data without
     creating extra objects.
    </para>