[SPARK-31333][SQL][DOCS] Document Join Hints
### What changes were proposed in this pull request? Document Join Hints ### Why are the changes needed? To make SQL Reference complete ### Does this PR introduce any user-facing change? Yes <img width="1049" alt="Screen Shot 2020-04-03 at 9 20 15 AM" src="https://user-images.githubusercontent.com/13592258/78382976-7c546b80-758c-11ea-9a8e-e46cfb7106f5.png"> <img width="1051" alt="Screen Shot 2020-04-03 at 10 39 55 AM" src="https://user-images.githubusercontent.com/13592258/78389778-356c7300-7598-11ea-8e6c-3742dadda11c.png"> ### How was this patch tested? Manually build and check Closes #28113 from huaxingao/join-hints. Authored-by: Huaxin Gao <huaxing@us.ibm.com> Signed-off-by: Sean Owen <srowen@gmail.com>
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url: sql-ref-syntax-qry-select-distribute-by.html
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- text: LIMIT Clause
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url: sql-ref-syntax-qry-select-limit.html
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- text: Join Hints
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url: sql-ref-syntax-qry-select-hints.html
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- text: EXPLAIN
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url: sql-ref-syntax-qry-explain.html
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- text: Auxiliary Statements
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---
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layout: global
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title: Query Hints
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displayTitle: Query Hints
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title: Join Hints
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displayTitle: Join Hints
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license: |
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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---
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### Description
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**This page is under construction**
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Join Hints allow users to suggest the join strategy that Spark should use. Prior to Spark 3.0, only the `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.
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### Join Hints Types
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<dl>
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<dt><code><em>BROADCAST</em></code></dt>
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<dd>
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Suggests that Spark use broadcast join. The join side with the hint will be broadcast regardless of <code>autoBroadcastJoinThreshold</code>. If both sides of the join have the broadcast hints, the one with the smaller size (based on stats) will be broadcast. The aliases for <code>BROADCAST</code> are <code>BROADCASTJOIN</code> and <code>MAPJOIN</code>.
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</dd>
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</dl>
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<dl>
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<dt><code><em>MERGE</em></code></dt>
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<dd>
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Suggests that Spark use shuffle sort merge join. The aliases for <code>MERGE</code> are <code>SHUFFLE_MERGE</code> and <code>MERGEJOIN</code>.
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</dd>
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</dl>
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<dl>
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<dt><code><em>SHUFFLE_HASH</em></code></dt>
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<dd>
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Suggests that Spark use shuffle hash join. If both sides have the shuffle hash hints, Spark chooses the smaller side (based on stats) as the build side.
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</dd>
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</dl>
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<dl>
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<dt><code><em>SHUFFLE_REPLICATE_NL</em></code></dt>
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<dd>
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Suggests that Spark use shuffle-and-replicate nested loop join.
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</dd>
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</dl>
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### Examples
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{% highlight sql %}
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-- Join Hints for broadcast join
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SELECT /*+ BROADCAST(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
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SELECT /*+ BROADCASTJOIN (t1) */ * FROM t1 left JOIN t2 ON t1.key = t2.key;
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SELECT /*+ MAPJOIN(t2) */ * FROM t1 right JOIN t2 ON t1.key = t2.key;
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-- Join Hints for shuffle sort merge join
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SELECT /*+ SHUFFLE_MERGE(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
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SELECT /*+ MERGEJOIN(t2) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
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SELECT /*+ MERGE(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
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-- Join Hints for shuffle hash join
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SELECT /*+ SHUFFLE_HASH(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
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-- Join Hints for shuffle-and-replicate nested loop join
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SELECT /*+ SHUFFLE_REPLICATE_NL(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
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-- When different join strategy hints are specified on both sides of a join, Spark
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-- prioritizes the BROADCAST hint over the MERGE hint over the SHUFFLE_HASH hint
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-- over the SHUFFLE_REPLICATE_NL hint.
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-- Spark will issue Warning in the following example
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-- org.apache.spark.sql.catalyst.analysis.HintErrorLogger: Hint (strategy=merge)
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-- is overridden by another hint and will not take effect.
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SELECT /*+ BROADCAST(t1) */ /*+ MERGE(t1, t2) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
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{% endhighlight %}
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