In the MySQL doc section on avoiding full table scans, one of the cases in which MySQL will use a full table scan is described like so:
You are using a key with low cardinality (many rows match the key value) through another column. In this case, MySQL assumes that by using the key it is likely to perform many key lookups and that a table scan would be faster.
I am having trouble understanding this.
For a starter, I'm not sure if I should parse the phrase as "using a key ... through another column" or "low cardinality ... through another column." Neither interpretation seems clear.
I can get the general idea that if I'm selecting a huge proportion of the table, like 75% or whatever, then the index will be slower to use (because of the dives to get the rows) than just reading the whole table. But I don't get what "through another column" has to do with it.
Can anyone explain this sentence?