[MM-36995] Start using db native JSON datatypes (#17930)

* Start using db native JSON datatypes

* Fix syntax to work on MySQL 5.7.12

store/sqlstore/store.go

@@ -911,7 +911,12 @@ func (ss *SqlStore) AlterColumnTypeIfExists(tableName string, columnName string,
 	if ss.DriverName() == model.DatabaseDriverMysql {
 		_, err = ss.GetMaster().ExecNoTimeout("ALTER TABLE " + tableName + " MODIFY " + columnName + " " + mySqlColType)
 	} else if ss.DriverName() == model.DatabaseDriverPostgres {
-		_, err = ss.GetMaster().ExecNoTimeout("ALTER TABLE " + strings.ToLower(tableName) + " ALTER COLUMN " + strings.ToLower(columnName) + " TYPE " + postgresColType)
+		query := "ALTER TABLE " + strings.ToLower(tableName) + " ALTER COLUMN " + strings.ToLower(columnName) + " TYPE " + postgresColType
+		// We need to explicitly cast when moving from text based to jsonb datatypes.
+		if postgresColType == "jsonb" {
+			query += " USING " + strings.ToLower(columnName) + "::jsonb"
+		}
+		_, err = ss.GetMaster().ExecNoTimeout(query)
 	}
 
 	if err != nil {
@@ -1742,3 +1747,10 @@ func versionString(v int, driver string) string {
 	}
 	return ""
 }
+
+func (ss *SqlStore) jsonDataType() string {
+	if ss.DriverName() == model.DatabaseDriverPostgres {
+		return "jsonb"
+	}
+	return "json"
+}