89354

Indexes

The MEMORY storage engine supports both HASH and BTREE indexes. You can specify one or the other

for a given index by adding a USING clause as shown here:

CREATE TABLE lookup

(id INT, INDEX USING HASH (id))

ENGINE = MEMORY;

CREATE TABLE lookup

(id INT, INDEX USING BTREE (id))

ENGINE - MEMORY;

For generał characteristics of B-tree and hash indexes, see Section 8.3.1, “How

MySQL Uses

lndexes".

MEMORY tables can have up to 64 indexes per table, 16 columns per index and a maximum key length of 3072 bytes.

If a MEMORY table hash index has a high degree of key duplication (many index entries containing the

same value), updates to the table that affect key values and all deletes are significantly slower. The

degree of this slowdown is proportional to the degree of duplication (or, inversely proportional to the

index cardinality). You can use a BTREE index to avoid this problem.

MEMORY tables can have nonunique keys. (This is an uncommon feature for

implementations of hash

indexes.)

Columns that are indexed can contain NULL values.

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