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

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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

BROADCAST

Suggests that Spark use broadcast join. The join side with the hint will be broadcast regardless of autoBroadcastJoinThreshold. 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 BROADCAST are BROADCASTJOIN and MAPJOIN.

MERGE

Suggests that Spark use shuffle sort merge join. The aliases for MERGE are SHUFFLE_MERGE and MERGEJOIN.

SHUFFLE_HASH

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.

SHUFFLE_REPLICATE_NL

Suggests that Spark use shuffle-and-replicate nested loop join.

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
• SELECT