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Displaying posts with tag: multiple-part index (reset)

Index merge annoyances fixed in MySQL 5.6
Employee +6 Vote Up -0Vote Down
While the index merge access types certainly are useful for a number of queries, there has been some frustration expressed both from customers and the community about how it...
  • is not used when it should have been
  • is used when ref access is obviously better
  • merges suboptimal indexes
  • is too restricted in which conditions can be used
  • I could come up with numerous examples of related bugs and feature requests dating back more than six years. To list a few: 17673, 30151, 23322,
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    On queries with many values in the IN clause
    Employee +4 Vote Up -0Vote Down
    A few customers with rather extreme needs have contacted us about a performance issue with the range optimizer. Our solution to the problem is to introduce a new variable in MySQL 5.6, eq_range_index_dive_limit, which can be used to control whether or not the range optimizer will a) do index dives, or b) use index statistics when estimating the number of rows in the ranges of the query. The former method gives a far more accurate estimate while the latter costs a lot less to compute.

    This is what the help text has to tell about the variable:

    The optimizer will use existing index statistics instead of doing index dives for equality ranges if the number of equality ranges for the index is larger than or equal to [the value of variable]. If set to 0, index dives are always used.
    "Equality range" means predicates using operators



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    Index Condition Pushdown to the rescue!
    Employee +2 Vote Up -0Vote Down
    A while ago, I explained how range access in a multiple-part index works and why MySQL can't utilize key parts beyond the first occurrence of some often used comparison operators. Luckily, there is a great improvement underway in MySQL 5.6 that will remedy much of this limitation. Meet Index Condition Pushdown.

    How does ICP work?

    Index Condition Pushdown is a new way for MySQL to evaluate conditions. Instead of evaluating conditions on rows read from a table, ICP makes it possible to evaluate conditions in the index and thereby avoid looking at the table if the condition is false.

    Let's assume that we have a multiple-part index covering columns (keypart_1, ..., keypart_n). Further assume that we have a condition with a





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    The MySQL range access method explained
    Employee +4 Vote Up -0Vote Down
    The range access method uses an index to read a subset of rows that form one or multiple continuous index value intervals. The intervals are defined by the query's range predicates, which are comparisons using any of =, <=>, IN(), IS NULL, IS NOT NULL, >, <, >=, <=, BETWEEN, !=, <> or LIKE.

    Some examples:
    SELECT * FROM blog WHERE author_id IN (1, 7, 8, 10)SELECT * FROM orders WHERE value > 1000
    You know that the range access method is used when EXPLAIN shows type=range.

    Naturally, there has to be an index on the column used by the range predicate. Since indexes are ordered, MySQL will, for each interval,





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    Showing entries 1 to 4

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