Allow "COPY table TO" command to copy rows from materialized views.

Previously, "COPY table TO" command worked only with plain tables and
did not support materialized views, even when they were populated and
had physical storage. To copy rows from materialized views,
"COPY (query) TO" command had to be used, instead.

This commit extends "COPY table TO" to support populated materialized
views directly, improving usability and performance, as "COPY table TO"
is generally faster than "COPY (query) TO". Note that copying from
unpopulated materialized views will still result in an error.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Vignesh C <vignesh21@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CACJufxHVxnyRYy67hiPePNCPwVBMzhTQ6FaL9_Te5On9udG=yg@mail.gmail.com

3 files changed, 29 insertions, 5 deletions

diff --git a/src/backend/commands/copyto.c b/src/backend/commands/copyto.c
index 84a3f3879a8..f87e405351d 100644
--- a/src/backend/commands/copyto.c
+++ b/src/backend/commands/copyto.c
@@ -653,11 +653,14 @@ BeginCopyTo(ParseState *pstate,
 							RelationGetRelationName(rel)),
 					 errhint("Try the COPY (SELECT ...) TO variant.")));
 		else if (rel->rd_rel->relkind == RELKIND_MATVIEW)
-			ereport(ERROR,
-					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-					 errmsg("cannot copy from materialized view \"%s\"",
-							RelationGetRelationName(rel)),
-					 errhint("Try the COPY (SELECT ...) TO variant.")));
+		{
+			if (!RelationIsPopulated(rel))
+				ereport(ERROR,
+						errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						errmsg("cannot copy from unpopulated materialized view \"%s\"",
+							   RelationGetRelationName(rel)),
+						errhint("Use the REFRESH MATERIALIZED VIEW command."));
+		}
 		else if (rel->rd_rel->relkind == RELKIND_FOREIGN_TABLE)
 			ereport(ERROR,
 					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
diff --git a/src/test/regress/expected/copy.out b/src/test/regress/expected/copy.out
index 06bae8c61ae..8d5a06563c4 100644
--- a/src/test/regress/expected/copy.out
+++ b/src/test/regress/expected/copy.out
@@ -338,3 +338,15 @@ create foreign table copytest_foreign_table (a int) server copytest_server;
 copy copytest_foreign_table from stdin (freeze);
 ERROR:  cannot perform COPY FREEZE on a foreign table
 rollback;
+-- Tests for COPY TO with materialized views.
+-- COPY TO should fail for an unpopulated materialized view
+-- but succeed for a populated one.
+CREATE MATERIALIZED VIEW copytest_mv AS SELECT 1 AS id WITH NO DATA;
+COPY copytest_mv(id) TO stdout WITH (header);
+ERROR:  cannot copy from unpopulated materialized view "copytest_mv"
+HINT:  Use the REFRESH MATERIALIZED VIEW command.
+REFRESH MATERIALIZED VIEW copytest_mv;
+COPY copytest_mv(id) TO stdout WITH (header);
+id
+1
+DROP MATERIALIZED VIEW copytest_mv;
diff --git a/src/test/regress/sql/copy.sql b/src/test/regress/sql/copy.sql
index 3009bdfdf89..f0b88a23db8 100644
--- a/src/test/regress/sql/copy.sql
+++ b/src/test/regress/sql/copy.sql
@@ -366,3 +366,12 @@ copy copytest_foreign_table from stdin (freeze);
 1
 \.
 rollback;
+
+-- Tests for COPY TO with materialized views.
+-- COPY TO should fail for an unpopulated materialized view
+-- but succeed for a populated one.
+CREATE MATERIALIZED VIEW copytest_mv AS SELECT 1 AS id WITH NO DATA;
+COPY copytest_mv(id) TO stdout WITH (header);
+REFRESH MATERIALIZED VIEW copytest_mv;
+COPY copytest_mv(id) TO stdout WITH (header);
+DROP MATERIALIZED VIEW copytest_mv;