NON-PRIMARY INDEXES

Problem: When adding a spatial index to a MyISAM table, the
cardinality of other indexes may become incorrect.

When doing ANALYZE TABLE, ha_myisam::analyze() calls chk_key() to
gather statistics about each index. Then, chk_keys() calls
update_key_parts() to generate a rows per key estimate for each key
part (i.e., column) of the index.

For B-tree indexes, there is one key part per column, and one key
segment for each key part. For R-trees, there is always just one
column and one key part, but four segments. The chk_key() and
update_key_parts() functions assume that there are as many key parts
as there are segments, and therefore generates four records per key
estimates for each R-tree instead of just one.

These estimates are then stored and later read back and assumed to be
the estimates of the three subsequent key parts, which are part of
other indexes. The estimate is then used to generate an estimate for
the index cardinality.

Fix: Write only one records per key estimate for each R-tree index.