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- Adaptive Query Execution
For some workloads, it is possible to improve performance by either caching data in memory, or byturning on some experimental options.
Caching Data In Memory
Spark SQL can cache tables using an in-memory columnar format by calling spark.catalog.cacheTable('tableName') or dataFrame.cache().Then Spark SQL will scan only required columns and will automatically tune compression to minimizememory usage and GC pressure. You can call spark.catalog.uncacheTable('tableName') to remove the table from memory.
Configuration of in-memory caching can be done using the setConf method on SparkSession or by runningSET key=value commands using SQL.
| Property Name | Default | Meaning | Since Version |
|---|---|---|---|
spark.sql.inMemoryColumnarStorage.compressed | true | When set to true Spark SQL will automatically select a compression codec for each column based on statistics of the data. | 1.0.1 |
spark.sql.inMemoryColumnarStorage.batchSize | 10000 | Controls the size of batches for columnar caching. Larger batch sizes can improve memory utilization and compression, but risk OOMs when caching data. | 1.1.1 |
Other Configuration Options
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The following options can also be used to tune the performance of query execution. It is possiblethat these options will be deprecated in future release as more optimizations are performed automatically.
| Property Name | Default | Meaning | Since Version |
|---|---|---|---|
spark.sql.files.maxPartitionBytes | 134217728 (128 MB) | The maximum number of bytes to pack into a single partition when reading files. This configuration is effective only when using file-based sources such as Parquet, JSON and ORC. | 2.0.0 |
spark.sql.files.openCostInBytes | 4194304 (4 MB) | The estimated cost to open a file, measured by the number of bytes could be scanned in the same time. This is used when putting multiple files into a partition. It is better to over-estimated, then the partitions with small files will be faster than partitions with bigger files (which is scheduled first). This configuration is effective only when using file-based sources such as Parquet, JSON and ORC. | 2.0.0 |
spark.sql.files.minPartitionNum | Default Parallelism | The suggested (not guaranteed) minimum number of split file partitions. If not set, the default value is `spark.default.parallelism`. This configuration is effective only when using file-based sources such as Parquet, JSON and ORC. | 3.1.0 |
spark.sql.broadcastTimeout | 300 | Timeout in seconds for the broadcast wait time in broadcast joins | 1.3.0 |
spark.sql.autoBroadcastJoinThreshold | 10485760 (10 MB) | Configures the maximum size in bytes for a table that will be broadcast to all worker nodes when performing a join. By setting this value to -1 broadcasting can be disabled. Note that currently statistics are only supported for Hive Metastore tables where the command ANALYZE TABLE COMPUTE STATISTICS noscan has been run. | 1.1.0 |
spark.sql.shuffle.partitions | 200 | Configures the number of partitions to use when shuffling data for joins or aggregations. | 1.1.0 |
spark.sql.sources.parallelPartitionDiscovery.threshold | 32 | Configures the threshold to enable parallel listing for job input paths. If the number of input paths is larger than this threshold, Spark will list the files by using Spark distributed job. Otherwise, it will fallback to sequential listing. This configuration is only effective when using file-based data sources such as Parquet, ORC and JSON. | 1.5.0 |
spark.sql.sources.parallelPartitionDiscovery.parallelism | 10000 | Configures the maximum listing parallelism for job input paths. In case the number of input paths is larger than this value, it will be throttled down to use this value. Same as above, this configuration is only effective when using file-based data sources such as Parquet, ORC and JSON. | 2.1.1 |
Join Strategy Hints for SQL Queries
The join strategy hints, namely BROADCAST, MERGE, SHUFFLE_HASH and SHUFFLE_REPLICATE_NL,instruct Spark to use the hinted strategy on each specified relation when joining them with anotherrelation. For example, when the BROADCAST hint is used on table ‘t1', broadcast join (eitherbroadcast hash join or broadcast nested loop join depending on whether there is any equi-join key)with ‘t1' as the build side will be prioritized by Spark even if the size of table ‘t1' suggestedby the statistics is above the configuration spark.sql.autoBroadcastJoinThreshold.
When different join strategy hints are specified on both sides of a join, Spark prioritizes theBROADCAST hint over the MERGE hint over the SHUFFLE_HASH hint over the SHUFFLE_REPLICATE_NLhint. When both sides are specified with the BROADCAST hint or the SHUFFLE_HASH hint, Spark willpick the build side based on the join type and the sizes of the relations.
Note that there is no guarantee that Spark will choose the join strategy specified in the hint sincea specific strategy may not support all join types.
For more details please refer to the documentation of Join Hints.
Coalesce Hints for SQL Queries
Coalesce hints allows the Spark SQL users to control the number of output files just like thecoalesce, repartition and repartitionByRange in Dataset API, they can be used for performancetuning and reducing the number of output files. The 'COALESCE' hint only has a partition number as aparameter. The 'REPARTITION' hint has a partition number, columns, or both of them as parameters.The 'REPARTITION_BY_RANGE' hint must have column names and a partition number is optional. Master of typing 2 v4 4 5.
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For more details please refer to the documentation of Partitioning Hints.
Adaptive Query Execution
Adaptive Query Execution (AQE) is an optimization technique in Spark SQL that makes use of the runtime statistics to choose the most efficient query execution plan. AQE is disabled by default. Spark SQL can use the umbrella configuration of spark.sql.adaptive.enabled to control whether turn it on/off. As of Spark 3.0, there are three major features in AQE, including coalescing post-shuffle partitions, converting sort-merge join to broadcast join, and skew join optimization.
Coalescing Post Shuffle Partitions
This feature coalesces the post shuffle partitions based on the map output statistics when both spark.sql.adaptive.enabled and spark.sql.adaptive.coalescePartitions.enabled configurations are true. This feature simplifies the tuning of shuffle partition number when running queries. You do not need to set a proper shuffle partition number to fit your dataset. Spark can pick the proper shuffle partition number at runtime once you set a large enough initial number of shuffle partitions via spark.sql.adaptive.coalescePartitions.initialPartitionNum configuration.
| Property Name | Default | Meaning | Since Version |
|---|---|---|---|
spark.sql.adaptive.coalescePartitions.enabled | true | When true and spark.sql.adaptive.enabled is true, Spark will coalesce contiguous shuffle partitions according to the target size (specified by spark.sql.adaptive.advisoryPartitionSizeInBytes), to avoid too many small tasks. | 3.0.0 |
spark.sql.adaptive.coalescePartitions.minPartitionNum | Default Parallelism | The minimum number of shuffle partitions after coalescing. If not set, the default value is the default parallelism of the Spark cluster. This configuration only has an effect when spark.sql.adaptive.enabled and spark.sql.adaptive.coalescePartitions.enabled are both enabled. | 3.0.0 |
spark.sql.adaptive.coalescePartitions.initialPartitionNum | (none) | The initial number of shuffle partitions before coalescing. If not set, it equals to spark.sql.shuffle.partitions. This configuration only has an effect when spark.sql.adaptive.enabled and spark.sql.adaptive.coalescePartitions.enabled are both enabled. | 3.0.0 |
spark.sql.adaptive.advisoryPartitionSizeInBytes | 64 MB | The advisory size in bytes of the shuffle partition during adaptive optimization (when spark.sql.adaptive.enabled is true). It takes effect when Spark coalesces small shuffle partitions or splits skewed shuffle partition. | 3.0.0 |
Converting sort-merge join to broadcast join
AQE converts sort-merge join to broadcast hash join when the runtime statistics of any join side is smaller than the broadcast hash join threshold. This is not as efficient as planning a broadcast hash join in the first place, but it's better than keep doing the sort-merge join, as we can save the sorting of both the join sides, and read shuffle files locally to save network traffic(if spark.sql.adaptive.localShuffleReader.enabled is true)
Optimizing Skew Join
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Data skew can severely downgrade the performance of join queries. This feature dynamically handles skew in sort-merge join by splitting (and replicating if needed) skewed tasks into roughly evenly sized tasks. It takes effect when both spark.sql.adaptive.enabled and spark.sql.adaptive.skewJoin.enabled configurations are enabled.
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| Property Name | Default | Meaning | Since Version |
|---|---|---|---|
spark.sql.adaptive.skewJoin.enabled | true | When true and spark.sql.adaptive.enabled is true, Spark dynamically handles skew in sort-merge join by splitting (and replicating if needed) skewed partitions. | 3.0.0 |
spark.sql.adaptive.skewJoin.skewedPartitionFactor | 5 | A partition is considered as skewed if its size is larger than this factor multiplying the median partition size and also larger than spark.sql.adaptive.skewJoin.skewedPartitionThresholdInBytes. | 3.0.0 |
spark.sql.adaptive.skewJoin.skewedPartitionThresholdInBytes | 256MB | A partition is considered as skewed if its size in bytes is larger than this threshold and also larger than spark.sql.adaptive.skewJoin.skewedPartitionFactor multiplying the median partition size. Ideally this config should be set larger than spark.sql.adaptive.advisoryPartitionSizeInBytes. | 3.0.0 |
