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mariadb still uses indexes with high average value group size.
I was playing around with indexes after we faced problems with
mariadb choosing the wrong index because of high average value group size.
So I created a table with 4 text columns and indexed the 4th column with
a prefix length of 1.
Following are the stats in brief --
The index has 35 unique values
There are around 1040000 rows in total.
this will give an average value group size of 29714
Cardinality is calculated as <no. of records in the index>/<average size
of value groups>, thus it's value must be 35
BUT from show indexes, it's value is shown as 72
(innodb_stats_persistent_sample_pages = 9999999).