Substring function is not parsed correctly

[theodesp]

I'm trying to implement #483 but it looks like we broke substring. The unit tests run fine but when we login into a console it doesn't work.

mysql> select SUBSTRING("foo", 2, 2);
ERROR 1105 (HY000): unknown error: syntax error at position 23 near 'foo'
mysql> select substring("foobar", 1, 2);
ERROR 1105 (HY000): unknown error: syntax error at position 26 near 'foobar'
mysql> select SUBSTRING("foo", 1, 2);
ERROR 1105 (HY000): unknown error: syntax error at position 23 near 'foo'

[erizocosmico]

This is probably an issue with vitess, because afaik we don't parse substring manually.

[kuba--]

It will require fix in vitess. So far, we get from vitess:

	// INVALID_ARGUMENT indicates client specified an invalid argument.
	// Note that this differs from FAILED_PRECONDITION. It indicates arguments
	// that are problematic regardless of the state of the system
	// (e.g., a malformed file name).
	Code_INVALID_ARGUMENT Code = 3

[kuba--]

Looks that vitess doesn't support SubstrExpr for static strings (only column names), so the fix is not super trivial just by updating vitess lexer but it will require changes in ast (by adding a new StaticSubstrExpr type or by extending current one SubstrExpr), then update lexer generator after that implement it in go-mysql-server parser.

[kuba--]

PR: vitessio/vitess#4350
But it will also require changes in /go-mysql-server.v0/sql/parse/parse.go by adding to func exprToExpression(e sqlparser.Expr):

	case *sqlparser.SubstrExpr:
		var (
			name sql.Expression
			err  error
		)
		if v.Name != nil {
			name, err = exprToExpression(v.Name)
		} else {
			name, err = exprToExpression(v.StrVal)
		}
//...