srowen commented on a change in pull request #28113: [SPARK-31333][SQL][DOCS]
Document Join Hints
URL: https://github.com/apache/spark/pull/28113#discussion_r403218675
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File path: docs/sql-ref-syntax-qry-select-hints.md
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@@ -1,22 +1,83 @@
---
layout: global
-title: Query Hints
-displayTitle: Query Hints
+title: Join Hints
+displayTitle: Join Hints
license: |
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.
---
+### Description
+
+Join Hints allow users to suggest the join strategy that Spark should use.
Prior to Spark 3.0, only `BROADCAST` Join Hint was supported. `MERGE`,
`SHUFFLE_HASH` and `SHUFFLE_REPLICATE_NL` Joint Hints support was added in 3.0.
When different join strategy hints are specified on both sides of a join, Spark
prioritizes hints in the following order: `BROADCAST` over `MERGE` over
`SHUFFLE_HASH` over `SHUFFLE_REPLICATE_NL`. When both sides are specified with
the `BROADCAST` hint or the `SHUFFLE_HASH` hint, Spark will pick the build side
based on the join type and the sizes of the relations. Since a given strategy
may not support all join types, Spark is not guaranteed to use the join
strategy suggested by the hint.
Review comment:
only BROADCAST Join Hint -> only the BROADCAST Join Hint ?
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