I've exported the results of an EXPLAIN run on a query. What I find confusing is that there's the key column listing out one of the indexes from the list of possible_keys(not shown in picture) however only the top row makes mention of Using index explicitely in the Extra column.
- What does this mean in the 2nd row, is it not using the index listed in the key column?
How should I interpret what the contents of the key column is about and how it is used?
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a. Should I interpret this as that index is used in the where stage of this query?