cloud-fan opened a new pull request #34823:
URL: https://github.com/apache/spark/pull/34823
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### What changes were proposed in this pull request?
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This is a performance regression since Spark 3.1, caused by
https://issues.apache.org/jira/browse/SPARK-32295
If you run the query in the JIRA ticket
```
Seq(
(1, "x", "x", "x", "x", "x", "x", "x", "x", "x", "x", "x", "x", "x", "x",
"x", "x", "x", "x", "x", "x")
).toDF()
.checkpoint() // or save and reload to truncate lineage
.createOrReplaceTempView("sub")
session.sql("""
SELECT
*
FROM
(
SELECT
EXPLODE( ARRAY( * ) ) result
FROM
(
SELECT
_1 a, _2 b, _3 c, _4 d, _5 e, _6 f, _7 g, _8 h, _9 i, _10 j, _11 k,
_12 l, _13 m, _14 n, _15 o, _16 p, _17 q, _18 r, _19 s, _20 t, _21 u
FROM
sub
)
)
WHERE
result != ''
""").show()
```
You will hit OOM. The reason is that:
1. We infer additional predicates with `Generate`. In this case, it's
`size(array(cast(_1#21 as string), _2#22, _3#23, ...) > 0`
2. Because of the cast, the `ConstantFolding` rule can't optimize this
`size(array(...))`.
3. We end up with a plan containing this part
```
+- Project [_1#21 AS a#106, _2#22 AS b#107, _3#23 AS c#108, _4#24 AS
d#109, _5#25 AS e#110, _6#26 AS f#111, _7#27 AS g#112, _8#28 AS h#113, _9#29 AS
i#114, _10#30 AS j#115, _11#31 AS k#116, _12#32 AS l#117, _13#33 AS m#118,
_14#34 AS n#119, _15#35 AS o#120, _16#36 AS p#121, _17#37 AS q#122, _18#38 AS
r#123, _19#39 AS s#124, _20#40 AS t#125, _21#41 AS u#126]
+- Filter (size(array(cast(_1#21 as string), _2#22, _3#23, _4#24,
_5#25, _6#26, _7#27, _8#28, _9#29, _10#30, _11#31, _12#32, _13#33, _14#34,
_15#35, _16#36, _17#37, _18#38, _19#39, _20#40, _21#41), true) > 0)
+- LogicalRDD [_1#21, _2#22, _3#23, _4#24, _5#25, _6#26, _7#27,
_8#28, _9#29, _10#30, _11#31, _12#32, _13#33, _14#34, _15#35, _16#36, _17#37,
_18#38, _19#39, _20#40, _21#41]
```
When calculating the constraints of the `Project`, we generate around 2^20
expressions, due to this code
```
var allConstraints = child.constraints
projectList.foreach {
case a @ Alias(l: Literal, _) =>
allConstraints += EqualNullSafe(a.toAttribute, l)
case a @ Alias(e, _) =>
// For every alias in `projectList`, replace the reference in
constraints by its attribute.
allConstraints ++= allConstraints.map(_ transform {
case expr: Expression if expr.semanticEquals(e) =>
a.toAttribute
})
allConstraints += EqualNullSafe(e, a.toAttribute)
case _ => // Don't change.
}
```
There are 3 issues here:
1. We may infer complicated predicates from `Generate`
2. `ConstanFolding` rule is too conservative. At least `Cast` has no side
effect with ANSI-off.
3. When calculating constraints, we should have a upper bound to avoid
generating too many expressions.
This fixes the first 2 issues, and leaves the third one for the future.
### Why are the changes needed?
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fix a performance issue
### Does this PR introduce _any_ user-facing change?
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no
### How was this patch tested?
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new tests, and run the query in JIRA ticket locally.
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