MySQL EXPLAIN: “Using index” vs. “Using index condition”

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An example explains it best:

SELECT Year, Make --- possibly more fields and/or from extra tables
FROM myUsedCarInventory
WHERE Make="Toyota" AND Year > '2006'

Assuming the Available indexes are:
  CarId
  VIN
  Make
  Make and Year

This query would EXPLAIN with ‘Using Index’ because it doesn’t need, at all, to “hit” the myUsedCarInventory table itself since the “Make and Year” index “cover” its need with regards to the elements of the WHERE clause that pertain to that table.

Now, imagine, we keep the query the same, but for the addition of a condition on the color

...
WHERE Make="Toyota" AND Year > '2006' AND Color="Red"

This query would likely EXPLAIN with ‘Using Index Condition’ (the ‘likely’, here is for the case that Toyota + year would not be estimated to be selective enough, and the optimizer may decide to just scan the table). This would mean that MySQL would FIRST use the index to resolve the Make + Year, and it would have to lookup the corresponding row in the table as well, only for the rows that satisfy the Make + Year conditions. That’s what is sometimes referred as “push down optimization“.

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