addresses #290
Note: there is currently no issue backfilling the ghost table when the
characterset changes, likely because it's a insert-into-select-from and
it all occurs within mysql.
However, when applying DML events (UPDATE, DELETE, etc) the values are
sprintf'd into a prepared statement and due to the possibility of
migrating text column data containing invalid characters in the
destination charset, a conversion step is often necessary.
For example, when migrating a table/column from latin1 to utf8mb4, the
latin1 column may contain characters that are invalid single-byte utf8
characters. Characters in the \x80-\xFF range are most common. When
written to utf8mb4 column without conversion, they fail as they do not
exist in the utf8 codepage.
Converting these texts/characters to the destination charset using
convert(? using {charset}) will convert appropriately and the
update/replace will succeed.
I only point out the "Note:" above because there are two tests added
for this: latin1text-to-utf8mb4 and latin1text-to-ut8mb4-insert
The former is a test that fails prior to this commit. The latter is a
test that succeeds prior to this comment. Both are affected by the code
in this commit.
convert text to original charset, then destination
converting text first to the original charset and then to the
destination charset produces the most consistent results, as inserting
the binary into a utf8-charset column may encounter an error if there is
no prior context of latin1 encoding.
mysql> select hex(convert(char(189) using utf8mb4));
+---------------------------------------+
| hex(convert(char(189) using utf8mb4)) |
+---------------------------------------+
| |
+---------------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> select hex(convert(convert(char(189) using latin1) using utf8mb4));
+-------------------------------------------------------------+
| hex(convert(convert(char(189) using latin1) using utf8mb4)) |
+-------------------------------------------------------------+
| C2BD |
+-------------------------------------------------------------+
1 row in set (0.00 sec)
as seen in this failure on 5.5.62
Error 1300: Invalid utf8mb4 character string: 'BD'; query=
replace /* gh-ost `test`.`_gh_ost_test_gho` */ into
`test`.`_gh_ost_test_gho`
(`id`, `t`)
values
(?, convert(? using utf8mb4))
this test assumes a latin1-encoded table with content containing bytes
in the \x80-\xFF, which are invalid single-byte characters in utf8 and
cannot be inserted in the altered table when the column containing these
characters is changed to utf8(mb4).
since these characters cannot be inserted, gh-ost fails.
