Address review comments

pkg/sql/opt/testutils/opttester/opt_tester.go

@@ -518,6 +518,11 @@ func New(catalog cat.Catalog, sql string) *OptTester {
 //
 //  - skip-race: skips the test if the race detector is enabled.
 //
+//  - set: sets the session setting for the given SQL statement, for example:
+//         build set=prefer_lookup_joins_for_fks=true
+//         DELETE FROM parent WHERE p = 3
+//         ----
+//
 func (ot *OptTester) RunCommand(tb testing.TB, d *datadriven.TestData) string {
 	// Allow testcases to override the flags.
 	for _, a := range d.CmdArgs {
@@ -867,7 +872,7 @@ func (f *Flags) Set(arg datadriven.CmdArg) error {
 		for _, val := range arg.Vals {
 			s := strings.Split(val, "=")
 			if len(s) != 2 {
-				return errors.Errorf("Expected both session variable name and value for set command")
+				return errors.Errorf("Expected both session variable name and value for set flag")
 			}
 			err := sql.SetSessionVariable(f.ot.ctx, f.ot.evalCtx, s[0], s[1])
 			if err != nil {

pkg/sql/vars.go

@@ -2227,8 +2227,10 @@ func init() {
 	}()
 }
 
-// SetSessionVariable sets a new value for session setting `varName` is the
+// SetSessionVariable sets a new value for session setting `varName` in the
 // session settings owned by `evalCtx`, returning an error if not successful.
+// This function should only be used for testing. For general-purpose code,
+// please use SessionAccessor.SetSessionVar instead.
 func SetSessionVariable(
 	ctx context.Context, evalCtx eval.Context, varName, varValue string,
 ) (err error) {