Add a function

To add a builtin function to TiDB the best practice is to look at MySQL first and try to implement the function in such a way that it is commpatible. Avoid adding functions that are already deprecated in MySQL or that might soon be deprecrated.

Here we will implement a HELLO() function that has one argument that is a string. For this you need a clone of the pingcap/tidb repository

sql> SELECT HELLO("world");
ERROR: 1305 (42000): FUNCTION test.hello does not exist

The first step is to define the name of the function in parser/ast/functions.go:

// List scalar function names.
const (
...
        Hello = "hello"
)

This links ast.Hello with "hello". Note that the lookup for the function is done with the lowercase name, so always use the lowercase name, otherwise it won't find the function.

The next step is to modify expression/builtin.go

var funcs = map[string]functionClass{
...
        ast.Hello: &helloFunctionClass{baseFunctionClass{ast.Hello, 1, 1}},
}

Now we need to define helloFunctionClass. We will do this in expression/builtin_string.go. The functions are organised in multiple files, pick the one that fits best.

var (
...
        _ functionClass = &helloFunctionClass{}
)
...
var (
        _ builtinFunc = &builtinHelloSig{}
)
...
type helloFunctionClass struct {
        baseFunctionClass
}

func (c *helloFunctionClass) getFunction(ctx BuildContext, args []Expression) (builtinFunc, error) {
        if err := c.verifyArgs(args); err != nil {
                return nil, err
        }
        bf, err := newBaseBuiltinFuncWithTp(ctx, c.funcName, args, types.ETString, types.ETString)
        if err != nil {
                return nil, err
        }
        sig := &builtinHelloSig{bf}
        return sig, nil
}

type builtinHelloSig struct {
        baseBuiltinFunc
}

func (b *builtinHelloSig) Clone() builtinFunc {
        newSig := &builtinHelloSig{}
        newSig.cloneFrom(&b.baseBuiltinFunc)
        return newSig
}

func (b *builtinHelloSig) evalString(ctx EvalContext, row chunk.Row) (name string, isNull bool, err error) {
        name, isNull, err = b.args[0].EvalString(ctx, row)
        if isNull || err != nil {
                return name, isNull, err
        }
        return "hello " + name, false, nil
}

The getFunction() method can return different functions depending on the type and number of arguments. This example always returns the same function that has one string argument and returns a string.

Here evalString() gets called for every row. If the function returns an integer you have to use evalInt and there are also functions for Decimal, Real, Time and JSON.

Now you need to build TiDB again and try the newly added function.

The final result:

sql> SELECT HELLO("world");
+----------------+
| HELLO("world") |
+----------------+
| hello world    |
+----------------+
1 row in set (0.0007 sec)

To show the function with multiple rows:

sql> WITH names AS (SELECT "Europe" AS "name" UNION ALL SELECT "America" UNION ALL SELECT "China")
   -> SELECT HELLO(name) FROM names;
+---------------+
| HELLO(name)   |
+---------------+
| hello Europe  |
| hello America |
| hello China   |
+---------------+
3 rows in set (0.0008 sec)

For testing have a look at expression/builtin_string_test.go.