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The 'default row' record (which stores the default values of
instantly added columns when they were added) must be ignored
during normal operation.

Because btr_pcur_store_position() can theoretically be invoked
on the pseudo-record, we must check every caller of
btr_pcur_restore_position(). Similarly, when positioning the
cursor to the start of an index with btr_pcur_open_at_index_side(),
or when moving to the previous record, we must be careful to
skip the 'default row' record.

rec_is_default_row(): Check whether a leaf page record is the
'default row' pseudo-record.

row_import_set_sys_max_row_id(): Check for a table that is empty but
contains the 'default row' record.

IndexPurge::open(), row_merge_read_clustered_index(),
dict_stats_analyze_index_level(), row_sel_try_search_shortcut(),
row_sel(), sel_restore_position_for_mysql(), row_search_mvcc():
Skip the 'default row' pseudo-record.

fts_get_max_doc_id(): Assert that the 'default row' cannot exist
in the secondary index FTS_DOC_ID_INDEX(FTS_DOC_ID).

page_find_rec_max_not_deleted(): Assert that we are operating
on a leaf page. Also skip the 'default row' pseudo-record, not
only delete-marked records.

row_search_get_max_rec(): Assert that page_find_rec_max_not_deleted()
did not return any delete-marked or 'default row' record.