expression rewriting: clean up CNF limits

Signed-off-by: Andres Taylor <[email protected]>

go/vt/sqlparser/predicate_rewriting.go

@@ -20,6 +20,10 @@ import (
 	"vitess.io/vitess/go/vt/log"
 )
 
+// This is the number of OR expressions in a predicate that will disable the CNF
+// rewrite because we don't want to send large queries to MySQL
+const CNFOrLimit = 5
+
 // RewritePredicate walks the input AST and rewrites any boolean logic into a simpler form
 // This simpler form is CNF plus logic for extracting predicates from OR, plus logic for turning ORs into IN
 // Note: In order to re-plan, we need to empty the accumulated metadata in the AST,
@@ -34,7 +38,7 @@ func RewritePredicate(ast SQLNode) SQLNode {
 		return true, nil
 	}, ast)
 
-	allowCNF := count < 4
+	allowCNF := count < CNFOrLimit
 
 	for {
 		printExpr(ast)

go/vt/sqlparser/predicate_rewriting_test.go

@@ -176,7 +176,7 @@ func TestExtractINFromOR(in *testing.T) {
 		in:       "(a in (1, 5) and B or C and a in (5, 7))",
 		expected: "a in (1, 5, 7)",
 	}, {
-		in:       "(a = 1 and b = 41) or (a = 2 and b = 42) or (a = 3 and b = 43) or (a = 4 and b = 44)",
+		in:       "(a = 5 and b = 1 or b = 2 and a = 6 or b = 3 and a = 4)",
 		expected: "<nil>",
 	}}