[SPARK-13827][SQL] Can't add subquery to an operator with same-name outputs while generate SQL string

## What changes were proposed in this pull request?

This PR tries to solve a fundamental issue in the `SQLBuilder`. When we want to turn a logical plan into SQL string and put it after FROM clause, we need to wrap it with a sub-query. However, a logical plan is allowed to have same-name outputs with different qualifiers(e.g. the `Join` operator), and this kind of plan can't be put under a subquery as we will erase and assign a new qualifier to all outputs and make it impossible to distinguish same-name outputs.

To solve this problem, this PR renames all attributes with globally unique names(using exprId), so that we don't need qualifiers to resolve ambiguity anymore.

For example, `SELECT x.key, MAX(y.key) OVER () FROM t x JOIN t y`, we will parse this SQL to a Window operator and a Project operator, and add a sub-query between them. The generated SQL looks like:
```
SELECT sq_1.key, sq_1.max
FROM (
    SELECT sq_0.key, sq_0.key, MAX(sq_0.key) OVER () AS max
    FROM (
        SELECT x.key, y.key FROM t1 AS x JOIN t2 AS y
    ) AS sq_0
) AS sq_1
```
You can see, the `key` columns become ambiguous after `sq_0`.

After this PR, it will generate something like:
```
SELECT attr_30 AS key, attr_37 AS max
FROM (
    SELECT attr_30, attr_37
    FROM (
        SELECT attr_30, attr_35, MAX(attr_35) AS attr_37
        FROM (
            SELECT attr_30, attr_35 FROM
                (SELECT key AS attr_30 FROM t1) AS sq_0
            INNER JOIN
                (SELECT key AS attr_35 FROM t1) AS sq_1
        ) AS sq_2
    ) AS sq_3
) AS sq_4
```
The outermost SELECT is used to turn the generated named to real names back, and the innermost SELECT is used to alias real columns to our generated names. Between them, there is no name ambiguity anymore.

## How was this patch tested?

existing tests and new tests in LogicalPlanToSQLSuite.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #11658 from cloud-fan/gensql.

1 files changed, 2 insertions, 4 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala
index 1af5437647..271ef33090 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/namedExpressions.scala
@@ -185,8 +185,7 @@ case class Alias(child: Expression, name: String)(
   override def sql: String = {
     val qualifiersString =
       if (qualifiers.isEmpty) "" else qualifiers.map(quoteIdentifier).mkString("", ".", ".")
-    val aliasName = if (isGenerated) s"$name#${exprId.id}" else s"$name"
-    s"${child.sql} AS $qualifiersString${quoteIdentifier(aliasName)}"
+    s"${child.sql} AS $qualifiersString${quoteIdentifier(name)}"
   }
 }
 
@@ -302,8 +301,7 @@ case class AttributeReference(
   override def sql: String = {
     val qualifiersString =
       if (qualifiers.isEmpty) "" else qualifiers.map(quoteIdentifier).mkString("", ".", ".")
-    val attrRefName = if (isGenerated) s"$name#${exprId.id}" else s"$name"
-    s"$qualifiersString${quoteIdentifier(attrRefName)}"
+    s"$qualifiersString${quoteIdentifier(name)}"
   }
 }