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When updating a table with virtual BLOB columns, the following might happen: - an old record is read from the table, it has no virtual blob values - update_virtual_fields() is run, vcol blob gets its value into the record. But only a pointer to the value is in the table->record[0], the value is in Field_blob::value String (but it doesn't have to be! it can be in the record, if the column is just a copy of another columns: ... b VARCHAR, c BLOB AS (b) ...) - store_record(table,record[1]), old record now is in record[1] - fill_record() prepares new values in record[0], vcol blob is updated, new value replaces the old one in the Field_blob::value - now both record[1] and record[0] have a pointer that points to the *new* vcol blob value. Or record[1] has a pointer to nowhere if Field_blob::value had to realloc. To resolve this we unlink vcol blobs from the pointer to the data (in the record[1]). Because the value is not *always* in the Field_blob::value String, we need to remember what blobs were unlinked. The orphan memory must be freed manually. To complicate the matter, ha_update_row() is also used in multi-update, in REPLACE, in INSERT ... ON DUPLICATE KEY UPDATE, also on REPLACE ... SELECT, REPLACE DELAYED, and LOAD DATA REPLACE, etc
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