sql: fix cached gists panic'ing after schema changes

Previously we didn't guard against out of bounds column ids in gists,
if a gist is created on a table and then that table drops columns we
would hit a runtime index out of bounds panic. By design gist decoding
is best effort and should silently ignore these columns when this
happens. The columns are used for equality conditions, ie:

             └── • lookup join
                 │ table: broker@broker_pkey
                 │ equality: (tr_s_symb) = (b_id)

So they are a nice to have. Doing anything more sophisticated would
dramatically increase the gist size so isn't worth it.

Fixes: #76800

Release note: None

pkg/sql/opt/exec/execbuilder/testdata/explain_gist

@@ -62,3 +62,25 @@ query T
 SELECT crdb_internal.decode_plan_gist('$gist')
 ----
 • <unknown>
+
+# Regression test for #76800
+statement ok
+CREATE TABLE t2 (a int, b int, c int, d int, e int)
+
+let $gist
+EXPLAIN (GIST) SELECT * FROM t2
+
+# To hit bug requires deleting lots of columns because of hidden columns.
+statement ok
+ALTER TABLE t2 DROP COLUMN b;
+ALTER TABLE t2 DROP COLUMN c;
+ALTER TABLE t2 DROP COLUMN a;
+ALTER TABLE t2 DROP COLUMN d;
+ALTER TABLE t2 DROP COLUMN e
+
+query T
+SELECT crdb_internal.decode_plan_gist('$gist')
+----
+• scan
+  table: t2@t2_pkey
+  spans: FULL SCAN

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

@@ -201,11 +201,15 @@ func getResultColumns(
 func tableColumns(table cat.Table, ordinals exec.TableColumnOrdinalSet) colinfo.ResultColumns {
 	cols := make(colinfo.ResultColumns, 0, ordinals.Len())
 	for i, ok := ordinals.Next(0); ok; i, ok = ordinals.Next(i + 1) {
-		col := table.Column(i)
-		cols = append(cols, colinfo.ResultColumn{
-			Name: string(col.ColName()),
-			Typ:  col.DatumType(),
-		})
+		// Be defensive about bitset values because they may come from cached
+		// gists and the columns they refer to could have been removed.
+		if i < table.ColumnCount() {
+			col := table.Column(i)
+			cols = append(cols, colinfo.ResultColumn{
+				Name: string(col.ColName()),
+				Typ:  col.DatumType(),
+			})
+		}
 	}
 	return cols
 }

pkg/sql/plan_opt.go

@@ -655,8 +655,7 @@ func (opc *optPlanningCtx) runExecBuilder(
 	return nil
 }
 
-// DecodeGist Avoid an import cycle by keeping the cat out of the tree, RFC: is
-// there a better way?
+// DecodeGist Avoid an import cycle by keeping the cat out of the tree.
 func (p *planner) DecodeGist(gist string) ([]string, error) {
 	return explain.DecodePlanGistToRows(gist, &p.optPlanningCtx.catalog)
 }