## What changes were proposed in this pull request?
This PR proposes a new optimization rule that replaces `Literal(null, _)` with `FalseLiteral` in conditions in `Join` and `Filter`, predicates in `If`, conditions in `CaseWhen`.
The idea is that some expressions evaluate to `false` if the underlying expression is `null` (as an example see `GeneratePredicate$create` or `doGenCode` and `eval` methods in `If` and `CaseWhen`). Therefore, we can replace `Literal(null, _)` with `FalseLiteral`, which can lead to more optimizations later on.
Let’s consider a few examples.
```
val df = spark.range(1, 100).select($"id".as("l"), ($"id" > 50).as("b"))
df.createOrReplaceTempView("t")
df.createOrReplaceTempView("p")
```
**Case 1**
```
spark.sql("SELECT * FROM t WHERE if(l > 10, false, NULL)").explain(true)
// without the new rule
…
== Optimized Logical Plan ==
Project [id#0L AS l#2L, cast(id#0L as string) AS s#3]
+- Filter if ((id#0L > 10)) false else null
+- Range (1, 100, step=1, splits=Some(12))
== Physical Plan ==
*(1) Project [id#0L AS l#2L, cast(id#0L as string) AS s#3]
+- *(1) Filter if ((id#0L > 10)) false else null
+- *(1) Range (1, 100, step=1, splits=12)
// with the new rule
…
== Optimized Logical Plan ==
LocalRelation <empty>, [l#2L, s#3]
== Physical Plan ==
LocalTableScan <empty>, [l#2L, s#3]
```
**Case 2**
```
spark.sql("SELECT * FROM t WHERE CASE WHEN l < 10 THEN null WHEN l > 40 THEN false ELSE null END”).explain(true)
// without the new rule
...
== Optimized Logical Plan ==
Project [id#0L AS l#2L, cast(id#0L as string) AS s#3]
+- Filter CASE WHEN (id#0L < 10) THEN null WHEN (id#0L > 40) THEN false ELSE null END
+- Range (1, 100, step=1, splits=Some(12))
== Physical Plan ==
*(1) Project [id#0L AS l#2L, cast(id#0L as string) AS s#3]
+- *(1) Filter CASE WHEN (id#0L < 10) THEN null WHEN (id#0L > 40) THEN false ELSE null END
+- *(1) Range (1, 100, step=1, splits=12)
// with the new rule
...
== Optimized Logical Plan ==
LocalRelation <empty>, [l#2L, s#3]
== Physical Plan ==
LocalTableScan <empty>, [l#2L, s#3]
```
**Case 3**
```
spark.sql("SELECT * FROM t JOIN p ON IF(t.l > p.l, null, false)").explain(true)
// without the new rule
...
== Optimized Logical Plan ==
Join Inner, if ((l#2L > l#37L)) null else false
:- Project [id#0L AS l#2L, cast(id#0L as string) AS s#3]
: +- Range (1, 100, step=1, splits=Some(12))
+- Project [id#0L AS l#37L, cast(id#0L as string) AS s#38]
+- Range (1, 100, step=1, splits=Some(12))
== Physical Plan ==
BroadcastNestedLoopJoin BuildRight, Inner, if ((l#2L > l#37L)) null else false
:- *(1) Project [id#0L AS l#2L, cast(id#0L as string) AS s#3]
: +- *(1) Range (1, 100, step=1, splits=12)
+- BroadcastExchange IdentityBroadcastMode
+- *(2) Project [id#0L AS l#37L, cast(id#0L as string) AS s#38]
+- *(2) Range (1, 100, step=1, splits=12)
// with the new rule
...
== Optimized Logical Plan ==
LocalRelation <empty>, [l#2L, s#3, l#37L, s#38]
```
## How was this patch tested?
This PR comes with a set of dedicated tests.
Closes#22857 from aokolnychyi/spark-25860.
Authored-by: Anton Okolnychyi <aokolnychyi@apple.com>
Signed-off-by: DB Tsai <d_tsai@apple.com>
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