[Columnar] Check for existence of Citus before creating Citus_Columnar (#6178)

* Added a check to see if Citus has already been loaded before creating citus_columnar

* added tests

src/backend/distributed/commands/extension.c

@@ -778,7 +778,7 @@ PreprocessCreateExtensionStmtForCitusColumnar(Node *parsetree)
 		}
 
 		/*citus version >= 11.1 requires install citus_columnar first*/
-		if (versionNumber >= 1110)
+		if (versionNumber >= 1110 && !CitusHasBeenLoaded())
 		{
 			if (get_extension_oid("citus_columnar", true) == InvalidOid)
 			{

src/test/regress/expected/multi_extension.out

@@ -1095,6 +1095,11 @@ SELECT * FROM multi_extension.print_extension_changes();
 ---------------------------------------------------------------------
 (0 rows)
 
+-- Test CREATE EXTENSION when Citus already exists but Citus_Columnar does not. Should skip
+CREATE EXTENSION IF NOT EXISTS citus;
+NOTICE:  extension "citus" already exists, skipping
+CREATE EXTENSION citus;
+ERROR:  extension "citus" already exists
 -- Snapshot of state at 11.1-1
 ALTER EXTENSION citus UPDATE TO '11.1-1';
 SELECT * FROM multi_extension.print_extension_changes();

src/test/regress/sql/multi_extension.sql

@@ -500,6 +500,9 @@ ALTER EXTENSION citus UPDATE TO '11.0-4';
 -- Should be empty result since upgrade+downgrade should be a no-op
 SELECT * FROM multi_extension.print_extension_changes();
 
+-- Test CREATE EXTENSION when Citus already exists but Citus_Columnar does not. Should skip
+CREATE EXTENSION IF NOT EXISTS citus;
+CREATE EXTENSION citus;
 -- Snapshot of state at 11.1-1
 ALTER EXTENSION citus UPDATE TO '11.1-1';
 SELECT * FROM multi_extension.print_extension_changes();