14.2.5.12. Increase VARCHAR Size In-Place

VARCHAR size may be increased using an in-place ALTER TABLE, as in this example:

ALTER TABLE t1 ALGORITHM=INPLACE, CHANGE COLUMN c1 c1 VARCHAR(255);

This is true as long as the number of length bytes required by a VARCHAR column remains the same. For VARCHAR values of 0 to 255, one length byte is required to encode the value. For VARCHAR values of 256 bytes or more, two length bytes are required. As a result, in-place ALTER TABLE only supports increasing VARCHAR size from 0 to 255 bytes or increasing VARCHAR size from a value equal to or greater than 256 bytes.

In-place ALTER TABLE does not support increasing VARCHAR size from less than 256 bytes to a value equal to or greater than 256 bytes. In this case, the number of required length bytes would change from 1 to 2, which is only supported by a table copy (ALGORITHM=COPY). For example, attempting to change VARCHAR column size from 255 to 256 using in-place ALTER TABLE would return an error:

ALTER TABLE t1 ALGORITHM=INPLACE, CHANGE COLUMN c1 c1 VARCHAR(256);ERROR 0A000: ALGORITHM=INPLACE is not supported. Reason: Cannot changecolumn type INPLACE. Try ALGORITHM=COPY.

Decreasing VARCHAR size using in-place ALTER TABLE is not supported. Decreasing VARCHAR size requires a table copy (ALGORITHM=COPY).