Histogram-Based Statistics

MariaDB starting with 10.4.3

Histograms are collected by default from MariaDB 10.4.3.

Contents

1. System Variables
2. Example
3. See Also

Histogram-based statistics are a mechanism to improve the query plan chosen by the optimizer in certain situations. Before their introduction, all conditions on non-indexed columns were ignored when searching for the best execution plan. Histograms can be collected for both indexed and non-indexed columns, and are made available to the optimizer.

Histogram statistics are stored in the mysql.column_stats table, which stores data for engine-independent table statistics, and so are essentially a subset of engine-independent table statistics.

Consider this example, using the following query:

```SELECT * FROM t1,t2 WHERE t1.a=t2.a and t2.b BETWEEN 1 AND 3;
```

Let's assume that

• table t1 contains 100 records
• table t2 contains 1000 records
• there is a primary index on t1(a)
• there is a secondary index on t2(a)
• there is no index defined on column t2.b
• the selectivity of the condition t2.b BETWEEN (1,3) is high (~ 1%)

Before histograms were introduced, the optimizer would choose the plan that:

• accesses t1 using a table scan
• accesses t2 using index t2(a)
• checks the condition t2.b BETWEEN 1 AND 3

This plan examines all rows of both tables and performs 100 index look-ups.

With histograms available, the optimizer can choose the following, more efficient plan:

• accesses table t2 in a table scan
• checks the condition t2.b BETWEEN 1 AND 3
• accesses t1 using index t1(a)

This plan also examine all rows from t2, but it performs only 10 look-ups to access 10 rows of table t1.

System Variables

There are a number of system variables that affect histograms.

histogram_size

The histogram_size variable determines the size, in bytes, from 0 to 255, used for a histogram. This is effectively the number of bins for `histogram_type=SINGLE_PREC_HB` or number of bins/2 for `histogram_type=DOUBLE_PREC_HB`. If it is set to 0 (the default for MariaDB 10.4.2 and below), no histograms are created when running an ANALYZE TABLE.

histogram_type

The histogram_type variable determines whether single precision (`SINGLE_PREC_HB`) or double precision (`DOUBLE_PREC_HB`) height-balanced histograms are created. From MariaDB 10.4.3, double precision is the default. For MariaDB 10.4.2 and below, single precision is the default.

From MariaDB 10.8, `JSON_HB`, JSON-format histograms, are accepted.

optimizer_use_condition_selectivity

The optimizer_use_condition_selectivity controls which statistics can be used by the optimizer when looking for the best query execution plan.

• `1` Use selectivity of predicates as in MariaDB 5.5.
• `2` Use selectivity of all range predicates supported by indexes.
• `3` Use selectivity of all range predicates estimated without histogram.
• `4` Use selectivity of all range predicates estimated with histogram.
• `5` Additionally use selectivity of certain non-range predicates calculated on record sample.

From MariaDB 10.4.1, the default is `4`. Until MariaDB 10.4.0, the default is `1`.

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