stored columns (BLOBs).

btr_copy_blob_prefix(), btr_copy_zblob_prefix(),
btr_copy_externally_stored_field_prefix_low(),
btr_copy_externally_stored_field_prefix(),
btr_copy_externally_stored_field(),
btr_rec_copy_externally_stored_field():
Note that the page containing the clustered index record that points to
the BLOB must be latched.

btr_copy_zblob_prefix(): Note that there is no latch on the page, and thus
all accesses to a given page via this function must be covered by the same
set of locks or latches.

btr_copy_zblob_prefix(): Note that the block acquired by
buf_page_get_zip() is protected by an exclusive table lock or
or by a latch on the clustered index record.