sql: introduce a command to convert a database into a schema

Fixes cockroachdb#50885.

This commit introduces a command to convert a database into a user
defined schema under a desired database. This command can be used by
users who are currently emulating a Postgres style set of schemas in
CockroachDB with separate databases.

Release note (sql change): Users can now convert existing databases into
schemas under other databases through the
`ALTER DATABASE ... CONVERT TO SCHEMA UNDER PARENT ...` command. This
command can only be run by `admin` and is only valid for databases that
don't already have any child schemas other than `public`.

docs/generated/sql/bnf/stmt_block.bnf

@@ -739,6 +739,7 @@ unreserved_keyword ::=
 	| 'CONFIGURE'
 	| 'CONSTRAINTS'
 	| 'CONVERSION'
+	| 'CONVERT'
 	| 'COPY'
 	| 'COVERING'
 	| 'CREATEROLE'
@@ -1111,6 +1112,7 @@ alter_sequence_stmt ::=
 alter_database_stmt ::=
 	alter_rename_database_stmt
 	| alter_zone_database_stmt
+	| alter_database_to_schema_stmt
 
 alter_range_stmt ::=
 	alter_zone_range_stmt
@@ -1522,6 +1524,9 @@ alter_rename_database_stmt ::=
 alter_zone_database_stmt ::=
 	'ALTER' 'DATABASE' database_name set_zone_config
 
+alter_database_to_schema_stmt ::=
+	'ALTER' 'DATABASE' database_name 'CONVERT' 'TO' 'SCHEMA' 'WITH' 'PARENT' database_name
+
 alter_zone_range_stmt ::=
 	'ALTER' 'RANGE' zone_name set_zone_config
 

pkg/sql/create_schema.go

@@ -126,8 +126,7 @@ func (*createSchemaNode) Next(runParams) (bool, error) { return false, nil }
 func (*createSchemaNode) Values() tree.Datums          { return tree.Datums{} }
 func (n *createSchemaNode) Close(ctx context.Context)  {}
 
-// CreateSchema creates a schema. Currently only works in IF NOT EXISTS mode,
-// for schemas that do in fact already exist.
+// CreateSchema creates a schema.
 func (p *planner) CreateSchema(ctx context.Context, n *tree.CreateSchema) (planNode, error) {
 	return &createSchemaNode{
 		n: n,

pkg/sql/drop_cascade.go

@@ -73,7 +73,7 @@ func (d *dropCascadeState) resolveCollectedObjects(
 			ctx,
 			tree.ObjectLookupFlags{
 				// Note we set required to be false here in order to not error out
-				// if we don't find the object,
+				// if we don't find the object.
 				CommonLookupFlags: tree.CommonLookupFlags{Required: false},
 				RequireMutable:    true,
 				IncludeOffline:    true,

pkg/sql/logictest/testdata/logic_test/reparent_database

@@ -0,0 +1,66 @@
+statement ok
+SET experimental_enable_user_defined_schemas = true;
+SET experimental_enable_enums = true;
+
+statement ok
+CREATE DATABASE pgdatabase;
+CREATE TABLE pgdatabase.t1 (x INT PRIMARY KEY);
+CREATE DATABASE pgschema;
+CREATE TABLE pgschema.t1 (x INT);
+CREATE TABLE pgschema.t2 (x INT);
+CREATE TABLE pgschema.t3 (x INT PRIMARY KEY);
+ALTER TABLE pgschema.t3 ADD FOREIGN KEY (x) REFERENCES pgdatabase.t1 (x); -- references shouldn't be affected by reparenting.
+CREATE TYPE pgschema.typ AS ENUM ('schema');
+
+let $db_id
+SELECT id FROM system.namespace WHERE name = 'pgschema'
+
+statement ok
+ALTER DATABASE pgschema CONVERT TO SCHEMA WITH PARENT pgdatabase
+
+query I
+SELECT * FROM pgdatabase.pgschema.t1
+
+query I
+SELECT * FROM pgdatabase.pgschema.t2
+
+query T
+SELECT 'schema'::pgdatabase.pgschema.typ
+----
+schema
+
+statement error pq: insert on table "t3" violates foreign key constraint "fk_x_ref_t1"
+INSERT INTO pgdatabase.pgschema.t3 VALUES (1)
+
+# Assert there aren't any namespace entries left with the old parentID.
+query T
+SELECT name FROM system.namespace WHERE "parentID" = $db_id
+
+# We can't reparent a database that has any child schemas.
+statement ok
+CREATE DATABASE parent;
+USE parent;
+CREATE SCHEMA child;
+USE test;
+
+statement error pq: cannot convert database with schemas into schema
+ALTER DATABASE parent CONVERT TO SCHEMA WITH PARENT pgdatabase
+
+# We can't reparent a database if it causes a name conflict.
+statement ok
+CREATE DATABASE pgschema
+
+statement error pq: schema "pgschema" already exists
+ALTER DATABASE pgschema CONVERT TO SCHEMA WITH PARENT pgdatabase
+
+statement ok
+DROP DATABASE pgschema
+
+# We can't convert a database with an invalid schema name into a schema.
+statement ok
+CREATE DATABASE pg_temp
+
+statement error pq: unacceptable schema name "pg_temp"
+ALTER DATABASE pg_temp CONVERT TO SCHEMA WITH PARENT pgdatabase
+
+

pkg/sql/opaque.go

@@ -117,6 +117,8 @@ func buildOpaque(
 		plan, err = p.RenameColumn(ctx, n)
 	case *tree.RenameDatabase:
 		plan, err = p.RenameDatabase(ctx, n)
+	case *tree.ReparentDatabase:
+		plan, err = p.ReparentDatabase(ctx, n)
 	case *tree.RenameIndex:
 		plan, err = p.RenameIndex(ctx, n)
 	case *tree.RenameTable:
@@ -213,6 +215,7 @@ func init() {
 		&tree.RenameDatabase{},
 		&tree.RenameIndex{},
 		&tree.RenameTable{},
+		&tree.ReparentDatabase{},
 		&tree.Revoke{},
 		&tree.RevokeRole{},
 		&tree.Scatter{},

pkg/sql/parser/sql.y

@@ -576,7 +576,7 @@ func (u *sqlSymUnion) executorType() tree.ScheduledJobExecutorType {
 %token <str> CHARACTER CHARACTERISTICS CHECK CLOSE
 %token <str> CLUSTER COALESCE COLLATE COLLATION COLUMN COLUMNS COMMENT COMMENTS COMMIT
 %token <str> COMMITTED COMPACT COMPLETE CONCAT CONCURRENTLY CONFIGURATION CONFIGURATIONS CONFIGURE
-%token <str> CONFLICT CONSTRAINT CONSTRAINTS CONTAINS CONVERSION COPY COVERING CREATE CREATEROLE
+%token <str> CONFLICT CONSTRAINT CONSTRAINTS CONTAINS CONVERSION CONVERT COPY COVERING CREATE CREATEROLE
 %token <str> CROSS CUBE CURRENT CURRENT_CATALOG CURRENT_DATE CURRENT_SCHEMA
 %token <str> CURRENT_ROLE CURRENT_TIME CURRENT_TIMESTAMP
 %token <str> CURRENT_USER CYCLE
@@ -714,6 +714,7 @@ func (u *sqlSymUnion) executorType() tree.ScheduledJobExecutorType {
 
 // ALTER DATABASE
 %type <tree.Statement> alter_rename_database_stmt
+%type <tree.Statement> alter_database_to_schema_stmt
 %type <tree.Statement> alter_zone_database_stmt
 
 // ALTER INDEX
@@ -1383,7 +1384,8 @@ alter_sequence_options_stmt:
 // %SeeAlso: WEBDOCS/alter-database.html
 alter_database_stmt:
   alter_rename_database_stmt
-|  alter_zone_database_stmt
+| alter_zone_database_stmt
+| alter_database_to_schema_stmt
 // ALTER DATABASE has its error help token here because the ALTER DATABASE
 // prefix is spread over multiple non-terminals.
 | ALTER DATABASE error // SHOW HELP: ALTER DATABASE
@@ -6564,6 +6566,12 @@ opt_asc_desc:
     $$.val = tree.DefaultDirection
   }
 
+alter_database_to_schema_stmt:
+  ALTER DATABASE database_name CONVERT TO SCHEMA WITH PARENT database_name
+  {
+    $$.val = &tree.ReparentDatabase{Name: tree.Name($3), Parent: tree.Name($9)}
+  }
+
 alter_rename_database_stmt:
   ALTER DATABASE database_name RENAME TO database_name
   {
@@ -11138,6 +11146,7 @@ unreserved_keyword:
 | CONFIGURE
 | CONSTRAINTS
 | CONVERSION
+| CONVERT
 | COPY
 | COVERING
 | CREATEROLE

pkg/sql/plan.go

@@ -192,6 +192,7 @@ var _ planNode = &renameColumnNode{}
 var _ planNode = &renameDatabaseNode{}
 var _ planNode = &renameIndexNode{}
 var _ planNode = &renameTableNode{}
+var _ planNode = &reparentDatabaseNode{}
 var _ planNode = &renderNode{}
 var _ planNode = &RevokeRoleNode{}
 var _ planNode = &rowCountNode{}
@@ -237,6 +238,7 @@ var _ planNodeReadingOwnWrites = &changePrivilegesNode{}
 var _ planNodeReadingOwnWrites = &dropSchemaNode{}
 var _ planNodeReadingOwnWrites = &dropTypeNode{}
 var _ planNodeReadingOwnWrites = &refreshMaterializedViewNode{}
+var _ planNodeReadingOwnWrites = &reparentDatabaseNode{}
 var _ planNodeReadingOwnWrites = &setZoneConfigNode{}
 
 // planNodeRequireSpool serves as marker for nodes whose parent must