The function spider_db_append_key_where_internal() converts
HA_READ_AFTER_KEY to '>'. The conversion seems to be correct for
single-column indexes because HA_READ_AFTER_KEY means "read the
key after the provided value."
However, how about multi-column indexes? Assume that there is a
multi-column index on c1 and c2 and we search with the condition
'c1 >= 100 AND c2 > 200'. The key_range.flag corresponds to the
search condition could be HA_READ_AFTER_KEY. In such a case,
we could not simply convert HA_READ_AFTER_KEY to '>'.
The correct conversion is to convert HA_READ_AFTER_KEY to '>'
only for the last column in key_part_map and to convert
HA_READ_AFTER_KEY to '>=' for the other column.
The similar discussion also applies to the conversion from
key_range.flag to a sign of inequality.
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