huaxingao commented on a change in pull request #28672:
URL: https://github.com/apache/spark/pull/28672#discussion_r432605518
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File path: docs/sql-ref-syntax-qry-select-hints.md
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@@ -21,14 +21,69 @@ license: |
### 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.
+Hints give users a way to suggest how Spark SQL to use specific approaches to
generate its execution plan.
### Syntax
```sql
-/*+ join_hint [ , ... ] */
+/*+ hint [ , ... ] */
```
+### Partitioning Hints
+
+`COALESCE`/`REPARTITION`/`REPARTITION_BY_RANGE` hints have functionalities
equivalent to those of the
+`Dataset` `coalesce`/`repartition`/`repartitionByRange` APIs. The `COALESCE`
hint can be used to reduce
+the number of partitions to the specified number of partitions. The
`REPARTITION`/`REPARTITION_BY_RANGE`
+hint can be used to repartition to the specified number of partitions using
the specified partitioning expressions.
+The `COALESCE` hint takes a partition number as a
+parameter. The `REPARTITION` hint takes a partition number, column names, or
both as parameters.
+The `REPARTITION_BY_RANGE` hint takes column names and an optional partition
number as parameters.
+These hints give users a way to tune performance and control the number of
output files in Spark SQL.
+
+### Examples
+```sql
+SELECT /*+ COALESCE(3) */ * FROM t;
+
+EXPLAIN SELECT /*+ COALESCE(3) */ * FROM t;
+== Physical Plan ==
+Coalesce 3
++- *(1) ColumnarToRow
+ +- FileScan parquet default.t[name#5,c#6] Batched: true, DataFilters: [],
Format: Parquet,
+ Location: CatalogFileIndex[file:/spark/spark-warehouse/t],
PartitionFilters: [],
+ PushedFilters: [], ReadSchema: struct<name:string>
+
+SELECT /*+ REPARTITION(3) */ * FROM t;
Review comment:
I think I will only have one example for explain. Otherwise the example
section will be too long.
I will leave the join hint example section as is for now. Don't want this
section to be too long.
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