Spark SQL 指南

提示

描述

提示为用户提供了一种建议 Spark SQL 如何使用特定方法来生成其执行计划的方式。

语法

/*+ hint [ , ... ] */

分区提示

分区提示允许用户建议 Spark 应遵循的分区策略。 支持COALESCE，REPARTITION和REPARTITION_BY_RANGE提示，并且分别等效于coalesce，repartition和repartitionByRange Dataset API。 REBALANCE只能用作提示。这些提示为用户提供了一种调整性能和控制Spark SQL中输出文件数量的方法。 如果指定了多个分区提示，则会将多个节点插入到逻辑计划中，但优化器会选择最左边的提示。

分区提示类型

示例

SELECT /*+ COALESCE(3) */ * FROM t;

SELECT /*+ REPARTITION(3) */ * FROM t;

SELECT /*+ REPARTITION(c) */ * FROM t;

SELECT /*+ REPARTITION(3, c) */ * FROM t;

SELECT /*+ REPARTITION_BY_RANGE(c) */ * FROM t;

SELECT /*+ REPARTITION_BY_RANGE(3, c) */ * FROM t;

SELECT /*+ REBALANCE */ * FROM t;

SELECT /*+ REBALANCE(3) */ * FROM t;

SELECT /*+ REBALANCE(c) */ * FROM t;

SELECT /*+ REBALANCE(3, c) */ * FROM t;

-- multiple partitioning hints
EXPLAIN EXTENDED SELECT /*+ REPARTITION(100), COALESCE(500), REPARTITION_BY_RANGE(3, c) */ * FROM t;
== Parsed Logical Plan ==
'UnresolvedHint REPARTITION, [100]
+- 'UnresolvedHint COALESCE, [500]
   +- 'UnresolvedHint REPARTITION_BY_RANGE, [3, 'c]
      +- 'Project [*]
         +- 'UnresolvedRelation [t]

== Analyzed Logical Plan ==
name: string, c: int
Repartition 100, true
+- Repartition 500, false
   +- RepartitionByExpression [c#30 ASC NULLS FIRST], 3
      +- Project [name#29, c#30]
         +- SubqueryAlias spark_catalog.default.t
            +- Relation[name#29,c#30] parquet

== Optimized Logical Plan ==
Repartition 100, true
+- Relation[name#29,c#30] parquet

== Physical Plan ==
Exchange RoundRobinPartitioning(100), false, [id=#121]
+- *(1) ColumnarToRow
   +- FileScan parquet default.t[name#29,c#30] Batched: true, DataFilters: [], Format: Parquet,
      Location: CatalogFileIndex[file:/spark/spark-warehouse/t], PartitionFilters: [],
      PushedFilters: [], ReadSchema: struct<name:string>

连接提示

连接提示允许用户建议 Spark 应使用的连接策略。 在 Spark 3.0 之前，仅支持BROADCAST连接提示。 在 3.0 中添加了MERGE，SHUFFLE_HASH和SHUFFLE_REPLICATE_NL联合提示支持。 如果在连接的两侧指定了不同的连接策略提示，则Spark会按以下顺序优先考虑提示：BROADCAST高于MERGE高于SHUFFLE_HASH高于SHUFFLE_REPLICATE_NL。 如果使用BROADCAST提示或SHUFFLE_HASH提示指定了两侧，则Spark将根据连接类型和关系的大小选择构建侧。 由于给定的策略可能不支持所有连接类型，因此不能保证 Spark 会使用提示建议的连接策略。

连接提示类型

示例

-- 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;