Allow charset introducers with vindex column values

[gedgar]

Feature Description

Currently, Vindex column values do not support charset introducers (or any expression for that matter).

This can be problematic if you have a BLOB column set as the from field of a lookup index and as a column in a lookup table for this vindex.

For example, attempting to:
INSERT INTO `Testing.GetByBytesEntity` (`bytes`) VALUES (@1); SELECT `id`, `bytes` FROM `Testing.GetByBytesEntity` WHERE `id`=last_insert_id();

Returns the following error:

vtgate: could not compute value for vindex or auto-inc column: expression is too complex '_binary :vtg19'

In this case, because parameter 1 is of type []byte, the go-sql-driver prepends the value with the _binary charset introducer. Attempts to work around this by sending the binary as a string-encoded equivalent (https://dev.mysql.com/doc/refman/5.7/en/hexadecimal-literals.html) were unsuccessful.

CC @harshit-gangal

[aquarapid]

The PR works for SELECT / INSERT; but fails on at least some UPDATE statements, viz:

mysql> desc t1;
+-----------+----------------+------+-----+---------+-------+
| Field     | Type           | Null | Key | Default | Extra |
+-----------+----------------+------+-----+---------+-------+
| id        | bigint         | NO   | PRI | NULL    |       |
| bytes_key | varbinary(255) | NO   | UNI | NULL    |       |
+-----------+----------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

• Note that column t1.bytes_key refers to a lookup vindex.

• Inserts work fine:

mysql> insert into t1 (id, bytes_key) values (1,_binary '\0\0');
Query OK, 1 row affected (0.03 sec)

• And so do selects, including selects with _binary:

mysql> select * from t1;
+----+----------------------+
| id | bytes_key            |
+----+----------------------+
|  1 | 0x0000               |
+----+----------------------+
1 row in set (0.01 sec)
mysql> select * from t1 where bytes_key = _binary '\0\0';
+----+----------------------+
| id | bytes_key            |
+----+----------------------+
|  1 | 0x0000               |
+----+----------------------+
1 row in set (0.01 sec)

• However, updates with _binary does not work:

mysql> update t1 set `bytes_key`=_binary '\0\001\002' where `id`=1;
ERROR 1235 (HY000): vtgate: http://localhost:15001/: unsupported: Only values are supported. Invalid update on column: bytes_key