RTREE
is an 'spatial index' that means that it can quickly identify 'close' values in
2 or more dimentions. Used in geographic databases for
queries like "all points within X meters from (x,y)"
HASH
is an unordered key-value map. It's even more efficient than BTREE: O(1) instead
of O(log n); but it doesn't have any concept of order. That means that is can't be used to avoid sort operations, or to fetch
ranges.
Originally, MySQL only allowed HASH indexes on
MEMORY tables; but i'm not sure if that has been
relaxed.
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