srowen closed pull request #22238: [SPARK-25245][DOCS][SS] Explain regarding
limiting modification on "spark.sql.shuffle.partitions" for structured streaming
URL: https://github.com/apache/spark/pull/22238
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diff --git a/docs/structured-streaming-programming-guide.md
b/docs/structured-streaming-programming-guide.md
index 73de1892977ac..8c3622c857240 100644
--- a/docs/structured-streaming-programming-guide.md
+++ b/docs/structured-streaming-programming-guide.md
@@ -2812,6 +2812,16 @@ See [Input Sources](#input-sources) and [Output
Sinks](#output-sinks) sections f
# Additional Information
+**Notes**
+
+- Several configurations are not modifiable after the query has run. To change
them, discard the checkpoint and start a new query. These configurations
include:
+ - `spark.sql.shuffle.partitions`
+ - This is due to the physical partitioning of state: state is partitioned
via applying hash function to key, hence the number of partitions for state
should be unchanged.
+ - If you want to run fewer tasks for stateful operations, `coalesce` would
help with avoiding unnecessary repartitioning.
+ - After `coalesce`, the number of (reduced) tasks will be kept unless
another shuffle happens.
+ - `spark.sql.streaming.stateStore.providerClass`: To read the previous state
of the query properly, the class of state store provider should be unchanged.
+ - `spark.sql.streaming.multipleWatermarkPolicy`: Modification of this would
lead inconsistent watermark value when query contains multiple watermarks,
hence the policy should be unchanged.
+
**Further Reading**
- See and run the
diff --git
a/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
b/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
index ef3ce98fd7add..ca100da9f019c 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
@@ -266,7 +266,9 @@ object SQLConf {
.createWithDefault(Long.MaxValue)
val SHUFFLE_PARTITIONS = buildConf("spark.sql.shuffle.partitions")
- .doc("The default number of partitions to use when shuffling data for
joins or aggregations.")
+ .doc("The default number of partitions to use when shuffling data for
joins or aggregations. " +
+ "Note: For structured streaming, this configuration cannot be changed
between query " +
+ "restarts from the same checkpoint location.")
.intConf
.createWithDefault(200)
@@ -868,7 +870,9 @@ object SQLConf {
.internal()
.doc(
"The class used to manage state data in stateful streaming queries.
This class must " +
- "be a subclass of StateStoreProvider, and must have a zero-arg
constructor.")
+ "be a subclass of StateStoreProvider, and must have a zero-arg
constructor. " +
+ "Note: For structured streaming, this configuration cannot be
changed between query " +
+ "restarts from the same checkpoint location.")
.stringConf
.createWithDefault(
"org.apache.spark.sql.execution.streaming.state.HDFSBackedStateStoreProvider")
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