sql: fix explain gist output to show number of scan span constraints

If there were span constraints we would always print 1, need to actually
append them to get the count right.

Fixes: cockroachdb#85324

Release note: None

pkg/sql/opt/exec/explain/plan_gist_factory.go

@@ -415,8 +415,10 @@ func (f *PlanGistFactory) decodeScanParams() exec.ScanParams {
 	if l > 0 {
 		idxConstraint = new(constraint.Constraint)
 		idxConstraint.Spans.Alloc(l)
-		var sp constraint.Span
-		idxConstraint.Spans.Append(&sp)
+		for i := 0; i < l; i++ {
+			var sp constraint.Span
+			idxConstraint.Spans.Append(&sp)
+		}
 	}
 
 	var invertedConstraint inverted.Spans

pkg/sql/opt/exec/explain/testdata/gists

@@ -1119,3 +1119,17 @@ explain(shape):
 • explain
 explain(gist):
 • explain
+
+# Regression test for #85324
+explain-plan-gist
+AgHyAQIA//8HHgAAByoFKiHyAQAA
+----
+• update
+│ table: ?
+│ set
+│
+└── • render
+    │
+    └── • scan
+          table: ?@?
+          spans: 15 spans