Re: pg_dump losing index column collations for unique and primary keys

I wrote:
> In short, I'd say the bug here is not pg_dump's fault at all,
> but failure to insist on collation match in ADD PRIMARY KEY
> USING INDEX.

Concretely, I think we should do the attached.

I'm not quite sure whether we should back-patch this, though.
It's been wrong since we added collations, but the main impact
of a back-patch might be to break cases that were working more
or less okay for people.

A compromise idea is to back-patch only into v12, where the issue
became quite a lot more important due to nondeterministic
collations.

Thoughts?

            regards, tom lane

diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index ee47547..b761fdf 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -2147,15 +2147,17 @@ transformIndexConstraint(Constraint *constraint, CreateStmtContext *cxt)
             if (i < index_form->indnkeyatts)
             {
                 /*
-                 * Insist on default opclass and sort options.  While the
-                 * index would still work as a constraint with non-default
-                 * settings, it might not provide exactly the same uniqueness
-                 * semantics as you'd get from a normally-created constraint;
-                 * and there's also the dump/reload problem mentioned above.
+                 * Insist on default opclass, collation, and sort options.
+                 * While the index would still work as a constraint with
+                 * non-default settings, it might not provide exactly the same
+                 * uniqueness semantics as you'd get from a normally-created
+                 * constraint; and there's also the dump/reload problem
+                 * mentioned above.
                  */
                 defopclass = GetDefaultOpClass(attform->atttypid,
                                                index_rel->rd_rel->relam);
                 if (indclass->values[i] != defopclass ||
+                    attform->attcollation != index_rel->rd_indcollation[i] ||
                     index_rel->rd_indoption[i] != 0)
                     ereport(ERROR,
                             (errcode(ERRCODE_WRONG_OBJECT_TYPE),
diff --git a/src/test/regress/expected/create_index.out b/src/test/regress/expected/create_index.out
index 1cdb7a9..645ae2c 100644
--- a/src/test/regress/expected/create_index.out
+++ b/src/test/regress/expected/create_index.out
@@ -1479,6 +1479,19 @@ primary key, btree, for table "public.cwi_test"
 DROP INDEX cwi_replaced_pkey;    -- Should fail; a constraint depends on it
 ERROR:  cannot drop index cwi_replaced_pkey because constraint cwi_replaced_pkey on table cwi_test requires it
 HINT:  You can drop constraint cwi_replaced_pkey on table cwi_test instead.
+-- Check that non-default index options are rejected
+CREATE UNIQUE INDEX cwi_uniq3_idx ON cwi_test(a desc);
+ALTER TABLE cwi_test ADD UNIQUE USING INDEX cwi_uniq3_idx;  -- fail
+ERROR:  index "cwi_uniq3_idx" column number 1 does not have default sorting behavior
+LINE 1: ALTER TABLE cwi_test ADD UNIQUE USING INDEX cwi_uniq3_idx;
+                                 ^
+DETAIL:  Cannot create a primary key or unique constraint using such an index.
+CREATE UNIQUE INDEX cwi_uniq4_idx ON cwi_test(b collate "POSIX");
+ALTER TABLE cwi_test ADD UNIQUE USING INDEX cwi_uniq4_idx;  -- fail
+ERROR:  index "cwi_uniq4_idx" column number 1 does not have default sorting behavior
+LINE 1: ALTER TABLE cwi_test ADD UNIQUE USING INDEX cwi_uniq4_idx;
+                                 ^
+DETAIL:  Cannot create a primary key or unique constraint using such an index.
 DROP TABLE cwi_test;
 -- ADD CONSTRAINT USING INDEX is forbidden on partitioned tables
 CREATE TABLE cwi_test(a int) PARTITION BY hash (a);
diff --git a/src/test/regress/sql/create_index.sql b/src/test/regress/sql/create_index.sql
index 7659808..73a55ea 100644
--- a/src/test/regress/sql/create_index.sql
+++ b/src/test/regress/sql/create_index.sql
@@ -538,6 +538,12 @@ ALTER TABLE cwi_test DROP CONSTRAINT cwi_uniq_idx,

 DROP INDEX cwi_replaced_pkey;    -- Should fail; a constraint depends on it

+-- Check that non-default index options are rejected
+CREATE UNIQUE INDEX cwi_uniq3_idx ON cwi_test(a desc);
+ALTER TABLE cwi_test ADD UNIQUE USING INDEX cwi_uniq3_idx;  -- fail
+CREATE UNIQUE INDEX cwi_uniq4_idx ON cwi_test(b collate "POSIX");
+ALTER TABLE cwi_test ADD UNIQUE USING INDEX cwi_uniq4_idx;  -- fail
+
 DROP TABLE cwi_test;

 -- ADD CONSTRAINT USING INDEX is forbidden on partitioned tables