spark-instrumented-optimizer/docs/sql-ref-syntax-qry-select-hints.md

---
layout: global
title: Join Hints
displayTitle: Join Hints
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---
### Description

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.

### Join Hints Types

<dl>
  <dt><code><em>BROADCAST</em></code></dt>
  <dd>
    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>.
  </dd>
</dl>

<dl>
  <dt><code><em>MERGE</em></code></dt>
  <dd>
     Suggests that Spark use shuffle sort merge join. The aliases for <code>MERGE</code> are <code>SHUFFLE_MERGE</code> and <code>MERGEJOIN</code>.
  </dd>
</dl>

<dl>
  <dt><code><em>SHUFFLE_HASH</em></code></dt>
  <dd>
     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.
  </dd>
</dl>

<dl>
  <dt><code><em>SHUFFLE_REPLICATE_NL</em></code></dt>
  <dd>
    Suggests that Spark use shuffle-and-replicate nested loop join.
  </dd>
</dl>

### Examples

{% highlight sql %}

-- Join Hints for broadcast join
SELECT /*+ BROADCAST(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
SELECT /*+ BROADCASTJOIN (t1) */ * FROM t1 left JOIN t2 ON t1.key = t2.key;
SELECT /*+ MAPJOIN(t2) */ * FROM t1 right JOIN t2 ON t1.key = t2.key;

-- Join Hints for shuffle sort merge join
SELECT /*+ SHUFFLE_MERGE(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
SELECT /*+ MERGEJOIN(t2) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;
SELECT /*+ MERGE(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;

-- Join Hints for shuffle hash join
SELECT /*+ SHUFFLE_HASH(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;

-- Join Hints for shuffle-and-replicate nested loop join
SELECT /*+ SHUFFLE_REPLICATE_NL(t1) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;

-- When different join strategy hints are specified on both sides of a join, Spark
-- prioritizes the BROADCAST hint over the MERGE hint over the SHUFFLE_HASH hint
-- over the SHUFFLE_REPLICATE_NL hint.
-- Spark will issue Warning in the following example
-- org.apache.spark.sql.catalyst.analysis.HintErrorLogger: Hint (strategy=merge)
-- is overridden by another hint and will not take effect.
SELECT /*+ BROADCAST(t1) */ /*+ MERGE(t1, t2) */ * FROM t1 INNER JOIN t2 ON t1.key = t2.key;

{% endhighlight %}

### Related Statements
- [JOIN](sql-ref-syntax-qry-select-join.html)
- [SELECT](sql-ref-syntax-qry-select.html)