sql/alter_primary_key: rewrite secondary index if new PK subsets old PK

Previously, during a `ALTER PRIMARY KEY`, if the new PK columns is a
subset of the old PK columns, we won't rewrite existing unique,
secondary index.

This was inadequate because the user might think this column is not used
anywhere and will want to drop it, which will unexpectedly drop the
dependent unique index.

Release note (bug fix): This PR fixed a bug where, in a `ALTER PRIMARY
KEY`, if the new PK columns is a subset of the old PK columns, we will
not rewrite existing secondary index, and hence those secondary indexes
continue to have some of the old PK columns in their `suffixColumns`.
But the user might, reasonably, think those columns are not used anymore
and proceed to drop them. The bug then caused those dependent secondary
indexes to be dropped, unexpectedly for the user.

pkg/sql/alter_primary_key.go

@@ -406,6 +406,11 @@ func (p *planner) AlterPrimaryKey(
 				return true, nil
 			}
 		}
+		if !idx.Primary() && catalog.MakeTableColSet(newPrimaryIndexDesc.KeyColumnIDs...).SubsetOf(
+			catalog.MakeTableColSet(tableDesc.PrimaryIndex.KeyColumnIDs...)) {
+			// Always rewrite a secondary index if the new PK columns is a (strict) subset of the old PK columns.
+			return true, nil
+		}
 		if idx.IsUnique() {
 			for i := 0; i < idx.NumKeyColumns(); i++ {
 				colID := idx.GetKeyColumnID(i)

pkg/sql/logictest/testdata/logic_test/alter_primary_key

@@ -1207,9 +1207,9 @@ t1_pkey        id     DESC
 t1_pkey        id2    N/A
 t1_pkey        name   N/A
 t1_pkey        rowid  N/A
+t1_id_key      id     ASC
 t1_id_id2_key  id     ASC
 t1_id_id2_key  id2    ASC
-t1_id_key      id     ASC
 
 
 statement ok
@@ -1222,9 +1222,9 @@ t1_pkey        id     DESC
 t1_pkey        id2    N/A
 t1_pkey        name   N/A
 t1_pkey        rowid  N/A
+t1_id_key      id     ASC
 t1_id_id2_key  id     ASC
 t1_id_id2_key  id2    ASC
-t1_id_key      id     ASC
 
 statement ok
 alter table t1 alter primary key using columns(id desc);
@@ -1236,9 +1236,9 @@ t1_pkey        id     DESC
 t1_pkey        id2    N/A
 t1_pkey        name   N/A
 t1_pkey        rowid  N/A
+t1_id_key      id     ASC
 t1_id_id2_key  id     ASC
 t1_id_id2_key  id2    ASC
-t1_id_key      id     ASC
 
 statement ok
 alter table t1 alter primary key using columns(id) USING HASH WITH (bucket_count=10)
@@ -1253,11 +1253,11 @@ t1_pkey        name                       N/A
 t1_pkey        rowid                      N/A
 t1_id_key1     id                         DESC
 t1_id_key1     crdb_internal_id_shard_10  ASC
+t1_id_key      id                         ASC
+t1_id_key      crdb_internal_id_shard_10  ASC
 t1_id_id2_key  id                         ASC
 t1_id_id2_key  id2                        ASC
 t1_id_id2_key  crdb_internal_id_shard_10  ASC
-t1_id_key      id                         ASC
-t1_id_key      crdb_internal_id_shard_10  ASC
 
 statement ok
 CREATE TABLE table_with_virtual_cols (
@@ -1637,3 +1637,72 @@ t_a_b_key  a  ASC  false
 t_a_b_key  b  ASC  false
 t_a_key    a  ASC  false
 t_a_key    b  ASC  true
+
+# The following regression test makes sure that when the new PK columns
+# is a (strict) subset of the old PK columns, all existing secondary indexes
+# were rewritten, and hence dropping a column from the old PK columns does not
+# unexpectedly drop an existing secondary index.
+subtest regression_#84040
+
+statement ok
+DROP TABLE IF EXISTS t
+
+statement ok
+CREATE TABLE t (
+  a INT NOT NULL,
+  b INT NOT NULL,
+  c INT NOT NULL,
+  PRIMARY KEY (a, b),
+  UNIQUE INDEX uniq_idx (c)
+);
+
+statement ok
+ALTER TABLE t ALTER PRIMARY KEY USING COLUMNS (a)
+
+statement ok
+ALTER TABLE t DROP COLUMN b
+
+query TTT
+select index_name,column_name,direction from [show indexes from t];
+----
+t_pkey    a  ASC
+t_pkey    c  N/A
+uniq_idx  c  ASC
+uniq_idx  a  ASC
+
+# Alter primary key on a hash-sharded primary key to be non-hash-sharded.
+# This had unexpectedly caused all unique indexes to be dropped silently on v21.2.
+# See the support issue https://github.com/cockroachlabs/support/issues/1687
+statement ok
+DROP TABLE IF EXISTS t;
+
+statement ok
+CREATE TABLE t (
+	i INT PRIMARY KEY USING HASH WITH (bucket_count=7) DEFAULT unique_rowid(),
+	j INT NOT NULL UNIQUE
+)
+
+# Assert that the primary key is hash-sharded and the unique index is created.
+query TTT
+SELECT index_name,column_name,direction FROM [SHOW INDEXES FROM t]
+----
+t_pkey      crdb_internal_i_shard_7  ASC
+t_pkey      i                        ASC
+t_pkey      j                        N/A
+t_j_key     j                        ASC
+t_j_key     crdb_internal_i_shard_7  ASC
+t_j_key     i                        ASC
+
+# Alter the primary key to be no longer hash-sharded.
+statement ok
+ALTER TABLE t ALTER PRIMARY KEY USING COLUMNS (i)
+
+# Now assert that the primary key has been modified to non-hash-sharded,
+# and the unique index still exists.
+query TTT
+SELECT index_name,column_name,direction FROM [SHOW INDEXES FROM t]
+----
+t_pkey      i            ASC
+t_pkey      j            N/A
+t_j_key     j            ASC
+t_j_key     i            ASC