[hotfix] Add a docs page with module specific guides for flink contributors

docs/content/docs/flinkDev/module_specific_guides.md

@@ -0,0 +1,57 @@
+---
+title: "Module specific guides"
+weight: 22
+type: docs
+aliases:
+- /flinkDev/module_specific_guides.html
+- /internals/module_specific_guides.html
+---
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# Module specific guides
+
+This page includes a list of guides, tips and tricks for contributing to specific Flink modules. 
+
+## Flink Table/SQL
+
+### Debug the planner
+
+When working on planner rules, It's useful to investigate which rules are applied, in which order and what transformation every rule generates. 
+In order to find out these details when running a planner test, you can enable the Calcite planner logging in debug mode adding the following lines to the log configuration in `flink-table/flink-table-planner/src/test/resources/log4j2-test.properties`:
+
+```
+loggers = testlogger
+logger.testlogger.name = org.apache.calcite.plan
+logger.testlogger.level = DEBUG
+logger.testlogger.appenderRefs = TestLogger
+```
+
+This is a sample output:
+
+```
+[...]
+3855 [main] DEBUG org.apache.calcite.plan.RelOptPlanner [] - call#1: Apply rule [FlinkJoinPushExpressionsRule] to [rel#40:LogicalJoin.NONE.any.None: 0.[NONE].[NONE](left=HepRelVertex#34,right=HepRelVertex#39,condition=AND(=($0, $5), >=($3, -($8, 3600000:INTERVAL HOUR)), <=($3, +($8, 3600000:INTERVAL HOUR))),joinType=inner)]
+3855 [main] DEBUG org.apache.calcite.plan.AbstractRelOptPlanner.rule_execution_summary [] - Rule Attempts Info for HepPlanner
+3856 [main] DEBUG org.apache.calcite.plan.AbstractRelOptPlanner.rule_execution_summary [] - 
+Rules                                                                   Attempts           Time (us)
+FlinkJoinPushExpressionsRule                                                   1                 143
+* Total                                                                        1                 143
+[...]
+```

flink-table/flink-table-planner/src/test/resources/log4j2-test.properties

@@ -21,8 +21,15 @@
 rootLogger.level = OFF
 rootLogger.appenderRef.test.ref = TestLogger
 
+appenders = testlogger
 appender.testlogger.name = TestLogger
 appender.testlogger.type = CONSOLE
 appender.testlogger.target = SYSTEM_ERR
 appender.testlogger.layout.type = PatternLayout
 appender.testlogger.layout.pattern = %-4r [%t] %-5p %c %x - %m%n
+
+# Uncomment the following lines to debug the planning
+#loggers = testlogger
+#logger.testlogger.name = org.apache.calcite.plan
+#logger.testlogger.level = DEBUG
+#logger.testlogger.appenderRefs = TestLogger