opt: do not project extra columns above partial index scans

The `GeneratePartialIndexScans` rule no longer projects all non-indexed
columns held constant by the partial index predicate. Now, only columns
produced by the original scan are produced. This fixes an issue where
the generated Project expression could have more output columns than the
other expressions in its group, causing a test-only assertion to fail.

Fixes #140019

There is no release note because this has caused no known correctness
bugs.

Release note: None

Author: mgartner

pkg/sql/opt/xform/scan_index_iter.go

@@ -306,8 +306,9 @@ func (it *scanIndexIter) ForEachStartingAfter(ord int, f enumerateIndexFunc) {
 				isCovering = true
 
 				// Build a projection only for constant columns not in the
-				// index.
-				constCols = constCols.Difference(indexCols)
+				// index and produced by the original scan.
+				constCols.DifferenceWith(indexCols)
+				constCols.IntersectionWith(it.scanPrivate.Cols)
 				constProj = it.buildConstProjectionsFromPredicate(predFilters, constCols)
 			}
 		}

pkg/sql/opt/xform/testdata/rules/select

@@ -557,6 +557,43 @@ project
       ├── columns: k:1!null
       └── key: (1)
 
+exec-ddl
+CREATE TABLE t140019 (
+  k INT PRIMARY KEY,
+  i INT,
+  j INT,
+  INDEX (k) WHERE i = 0 AND j = 1
+)
+----
+
+# Regression test for #140019. Do not project unnecessary columns, like i=0,
+# that are held constant by the partial index predicate.
+opt disable=SimplifyZeroCardinalityGroup
+SELECT NULL FROM t140019 WHERE false GROUP BY j
+----
+project
+ ├── columns: "?column?":6
+ ├── cardinality: [0 - 0]
+ ├── fd: ()-->(6)
+ ├── distinct-on
+ │    ├── columns: j:3
+ │    ├── grouping columns: j:3
+ │    ├── cardinality: [0 - 0]
+ │    ├── key: (3)
+ │    └── select
+ │         ├── columns: j:3
+ │         ├── cardinality: [0 - 0]
+ │         ├── project
+ │         │    ├── columns: j:3!null
+ │         │    ├── fd: ()-->(3)
+ │         │    ├── scan t140019@t140019_k_idx,partial
+ │         │    └── projections
+ │         │         └── 1 [as=j:3]
+ │         └── filters
+ │              └── false [constraints=(contradiction; tight)]
+ └── projections
+      └── NULL [as="?column?":6]
+
 # --------------------------------------------------
 # GenerateConstrainedScans
 # --------------------------------------------------