json: implement json unary not with comparing to json(0)

cmd/explaintest/r/select.result

@@ -658,3 +658,11 @@ insert into t3 values ('a');
 select * from t3 where a > 0x80;
 Error 1105 (HY000): Cannot convert string '\x80' from binary to utf8mb4
 set @@tidb_enable_outer_join_reorder=false;
+drop table if exists t1;
+create table t1(j json);
+insert into t1 values ('{"a":"b"}');
+explain format = 'brief' select * from t1 WHERE NOT t1.j;
+id	estRows	task	access object	operator info
+Selection	8000.00	root		eq(test.t1.j, "0")
+└─TableReader	10000.00	root		data:TableFullScan
+  └─TableFullScan	10000.00	cop[tikv]	table:t1	keep order:false, stats:pseudo

cmd/explaintest/t/select.test

@@ -283,3 +283,8 @@ insert into t3 values ('a');
 --error 1105
 select * from t3 where a > 0x80;
 set @@tidb_enable_outer_join_reorder=false;
+
+drop table if exists t1;
+create table t1(j json);
+insert into t1 values ('{"a":"b"}');
+explain format = 'brief' select * from t1 WHERE NOT t1.j;

errno/errcode.go

@@ -922,6 +922,7 @@ const (
 	ErrFunctionalIndexNotApplicable                          = 3909
 	ErrDynamicPrivilegeNotRegistered                         = 3929
 	ErUserAccessDeniedForUserAccountBlockedByPasswordLock    = 3955
+	ErrJSONInBooleanContext                                  = 3986
 	ErrTableWithoutPrimaryKey                                = 3750
 	// MariaDB errors.
 	ErrOnlyOneDefaultPartionAllowed         = 4030

errno/errname.go

@@ -924,6 +924,7 @@ var MySQLErrName = map[uint16]*mysql.ErrMessage{
 	ErrInvalidRequiresSingleReference:                        mysql.Message("In recursive query block of Recursive Common Table Expression '%s', the recursive table must be referenced only once, and not in any subquery", nil),
 	ErrCTEMaxRecursionDepth:                                  mysql.Message("Recursive query aborted after %d iterations. Try increasing @@cte_max_recursion_depth to a larger value", nil),
 	ErrTableWithoutPrimaryKey:                                mysql.Message("Unable to create or change a table without a primary key, when the system variable 'sql_require_primary_key' is set. Add a primary key to the table or unset this variable to avoid this message. Note that tables without a primary key can cause performance problems in row-based replication, so please consult your DBA before changing this setting.", nil),
+	ErrJSONInBooleanContext:                                  mysql.Message("Evaluating a JSON value in SQL boolean context does an implicit comparison against JSON integer 0; if this is not what you want, consider converting JSON to a SQL numeric type with JSON_VALUE RETURNING", nil),
 	// MariaDB errors.
 	ErrOnlyOneDefaultPartionAllowed:         mysql.Message("Only one DEFAULT partition allowed", nil),
 	ErrWrongPartitionTypeExpectedSystemTime: mysql.Message("Wrong partitioning type, expected type: `SYSTEM_TIME`", nil),

expression/builtin_op.go

@@ -20,6 +20,7 @@ import (
 	"strings"
 
 	"github.com/pingcap/errors"
+	"github.com/pingcap/tidb/parser/ast"
 	"github.com/pingcap/tidb/parser/mysql"
 	"github.com/pingcap/tidb/parser/opcode"
 	"github.com/pingcap/tidb/sessionctx"
@@ -718,7 +719,7 @@ func (c *unaryNotFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 	argTp := args[0].GetType().EvalType()
 	if argTp == types.ETTimestamp || argTp == types.ETDatetime || argTp == types.ETDuration {
 		argTp = types.ETInt
-	} else if argTp == types.ETJson || argTp == types.ETString {
+	} else if argTp == types.ETString {
 		argTp = types.ETReal
 	}
 
@@ -739,6 +740,17 @@ func (c *unaryNotFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 	case types.ETInt:
 		sig = &builtinUnaryNotIntSig{bf}
 		sig.setPbCode(tipb.ScalarFuncSig_UnaryNotInt)
+	case types.ETJson:
+		// not(json) is to compare the json with json integer 0
+		zeroJSON := &Constant{
+			Value:   types.NewJSONDatum(types.CreateBinaryJSON(int64(0))),
+			RetType: types.NewFieldType(mysql.TypeJSON),
+		}
+		sig, err = funcs[ast.EQ].getFunction(ctx, []Expression{args[0], zeroJSON})
+		if err != nil {
+			return nil, err
+		}
+		ctx.GetSessionVars().StmtCtx.AppendWarning(errJSONInBooleanContext)
 	default:
 		return nil, errors.Errorf("unexpected types.EvalType %v", argTp)
 	}

expression/builtin_op_test.go

@@ -462,6 +462,8 @@ func TestUnaryNot(t *testing.T) {
 		{[]interface{}{"0.3"}, 0, false, false},
 		{[]interface{}{types.NewDecFromFloatForTest(0.3)}, 0, false, false},
 		{[]interface{}{nil}, 0, true, false},
+		{[]interface{}{types.CreateBinaryJSON(int64(0))}, 1, false, false},
+		{[]interface{}{types.CreateBinaryJSON(map[string]interface{}{"test": "test"})}, 0, false, false},
 
 		{[]interface{}{errors.New("must error")}, 0, false, true},
 	}

expression/errors.go

@@ -60,6 +60,7 @@ var (
 	errSpecificAccessDenied          = dbterror.ClassExpression.NewStd(mysql.ErrSpecificAccessDenied)
 	errUserLockDeadlock              = dbterror.ClassExpression.NewStd(mysql.ErrUserLockDeadlock)
 	errUserLockWrongName             = dbterror.ClassExpression.NewStd(mysql.ErrUserLockWrongName)
+	errJSONInBooleanContext          = dbterror.ClassExpression.NewStd(mysql.ErrJSONInBooleanContext)
 
 	// Sequence usage privilege check.
 	errSequenceAccessDenied      = dbterror.ClassExpression.NewStd(mysql.ErrTableaccessDenied)

expression/scalar_function.go

@@ -234,6 +234,12 @@ func newFunctionImpl(ctx sessionctx.Context, fold int, funcName string, retType
 	if builtinRetTp := f.getRetTp(); builtinRetTp.GetType() != mysql.TypeUnspecified || retType.GetType() == mysql.TypeUnspecified {
 		retType = builtinRetTp
 	}
+	// TODO: add a method for functions to get a name, or use other ways to replace this special case
+	// not(json) is implemented with comparing json with json 0. If we don't overwrite the funcName,
+	// the explain information will be `not(json, "0")`. We prefer to make it `eq(json, "0")`
+	if funcName == ast.UnaryNot && funcArgs[0].GetType().GetType() == mysql.TypeJSON {
+		funcName = ast.EQ
+	}
 	sf := &ScalarFunction{
 		FuncName: model.NewCIStr(funcName),
 		RetType:  retType,