Cache the current database name

Purely for performance reasons.

src/backend/distributed/connection/connection_management.c

@@ -251,7 +251,7 @@ StartNodeUserDatabaseConnection(uint32 flags, const char *hostname, int32 port,
 	}
 	else
 	{
-		strlcpy(key.database, get_database_name(MyDatabaseId), NAMEDATALEN);
+		strlcpy(key.database, CurrentDatabaseName(), NAMEDATALEN);
 	}
 
 	if (CurrentCoordinatedTransactionState == COORD_TRANS_NONE)

src/backend/distributed/executor/multi_task_tracker_executor.c

@@ -871,7 +871,7 @@ TrackerConnectPoll(TaskTracker *taskTracker)
 		{
 			char *nodeName = taskTracker->workerName;
 			uint32 nodePort = taskTracker->workerPort;
-			char *nodeDatabase = get_database_name(MyDatabaseId);
+			char *nodeDatabase = CurrentDatabaseName();
 			char *nodeUser = taskTracker->userName;
 
 			int32 connectionId = MultiClientConnectStart(nodeName, nodePort,

src/backend/distributed/planner/multi_explain.c

@@ -507,7 +507,7 @@ ExplainTaskPlacement(ShardPlacement *taskPlacement, List *explainOutputList,
 	StringInfo nodeAddress = makeStringInfo();
 	char *nodeName = taskPlacement->nodeName;
 	uint32 nodePort = taskPlacement->nodePort;
-	char *nodeDatabase = get_database_name(MyDatabaseId);
+	char *nodeDatabase = CurrentDatabaseName();
 	ListCell *explainOutputCell = NULL;
 	int rowIndex = 0;
 

src/backend/distributed/utils/metadata_cache.c

@@ -24,6 +24,7 @@
 #include "catalog/pg_namespace.h"
 #include "catalog/pg_type.h"
 #include "citus_version.h"
+#include "commands/dbcommands.h"
 #include "commands/extension.h"
 #include "commands/trigger.h"
 #include "distributed/colocation_utils.h"
@@ -137,6 +138,8 @@ typedef struct MetadataCacheData
 	Oid unavailableNodeRoleId;
 	Oid pgTableIsVisibleFuncId;
 	Oid citusTableIsVisibleFuncId;
+	bool databaseNameValid;
+	char databaseName[NAMEDATALEN];
 } MetadataCacheData;
 
 
@@ -2102,6 +2105,33 @@ CitusTableVisibleFuncId(void)
 }
 
 
+/*
+ * CurrentDatabaseName gets the name of the current database and caches
+ * the result.
+ *
+ * Given that the database name cannot be changed when there is at least
+ * one session connected to it, we do not need to implement any invalidation
+ * mechanism.
+ */
+char *
+CurrentDatabaseName(void)
+{
+	if (!MetadataCache.databaseNameValid)
+	{
+		char *databaseName = get_database_name(MyDatabaseId);
+		if (databaseName == NULL)
+		{
+			return NULL;
+		}
+
+		strlcpy(MetadataCache.databaseName, databaseName, NAMEDATALEN);
+		MetadataCache.databaseNameValid = true;
+	}
+
+	return MetadataCache.databaseName;
+}
+
+
 /*
  * CitusExtensionOwner() returns the owner of the 'citus' extension. That user
  * is, amongst others, used to perform actions a normal user might not be

src/backend/distributed/worker/task_tracker_protocol.c

@@ -368,7 +368,7 @@ CreateTask(uint64 jobId, uint32 taskId, char *taskCallString)
 {
 	WorkerTask *workerTask = NULL;
 	uint32 assignmentTime = 0;
-	char *databaseName = get_database_name(MyDatabaseId);
+	char *databaseName = CurrentDatabaseName();
 	char *userName = CurrentUserName();
 
 	/* increase task priority for cleanup tasks */

src/backend/distributed/worker/worker_data_fetch_protocol.c

@@ -248,7 +248,7 @@ ReceiveRegularFile(const char *nodeName, uint32 nodePort, const char *nodeUser,
 	}
 
 	/* we use the same database name on the master and worker nodes */
-	nodeDatabase = get_database_name(MyDatabaseId);
+	nodeDatabase = CurrentDatabaseName();
 
 	/* connect to remote node */
 	connectionId = MultiClientConnect(nodeName, nodePort, nodeDatabase, nodeUser);

src/include/distributed/metadata_cache.h

@@ -167,4 +167,7 @@ extern Oid BinaryCopyFormatId(void);
 extern Oid CitusExtensionOwner(void);
 extern char * CitusExtensionOwnerName(void);
 extern char * CurrentUserName(void);
+extern char * CurrentDatabaseName(void);
+
+
 #endif /* METADATA_CACHE_H */