[MDEV-19285] INSTANT ALTER from ascii_general_ci to latin1_general_ci produces corrupt data - Jira

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Copying ALTER from ascii_general_ci to latin1_general_ci may fail in case of 8-bit characters:

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a VARCHAR(10) CHARACTER SET ascii COLLATE ascii_general_ci, PRIMARY KEY(a)) ENGINE=InnoDB;

INSERT INTO t1 VALUES ('a'),(0xC0),('b');

ALTER TABLE t1 ALGORITHM=COPY, MODIFY a VARCHAR(10) CHARACTER SET latin1 COLLATE latin1_general_ci;

Instant ALTER performs such conversion silently, but then CHECK TABLE rerpots corrupted data:

ALTER TABLE t1 ALGORITHM=INSTANT, MODIFY a VARCHAR(10) CHARACTER SET latin1 COLLATE latin1_general_ci;

CHECK TABLE t1;

+---------+-------+----------+---------------------------------------------------+

| Table   | Op    | Msg_type | Msg_text                                          |

+---------+-------+----------+---------------------------------------------------+

| test.t1 | check | Warning  | InnoDB: The B-tree of index PRIMARY is corrupted. |

| test.t1 | check | error    | Corrupt                                           |

+---------+-------+----------+---------------------------------------------------+

Instant ALTER should be disallowed from ascii_general_ci to xxx_general_ci.

is caused by

MDEV-15564 Avoid table rebuild in ALTER TABLE on collation or charset changes

relates to

MDEV-19284 INSTANT ALTER with ucs2-to-utf16 conversion produces bad data

MDEV-28323 Redundant Item_func_conv_charset on WHERE utf8mb4_field=utf8mb3_field

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