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Nikita Vasilev 2019-01-24 18:14:59 +03:00
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docs/en/operations/table_engines/mergetree.md
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@ -230,15 +230,15 @@ The key for partitioning by month allows reading only those data blocks which co
### Data Skipping Indices ### Data Skipping Indices
Index declaration in the columns section of create query. Index declaration in the columns section of the `CREATE` query.
```sql ```sql
INDEX index_name expr TYPE type(...) GRANULARITY granularity_value INDEX index_name expr TYPE type(...) GRANULARITY granularity_value
``` ```
For tables from the `*MergeTree` family data skipping indices can be specified. For tables from the `*MergeTree` family data skipping indices can be specified.
These indices aggregate some information about specified expression on blocks, which consist of `granularity_value` granules, These indices aggregate some information about the specified expression on blocks, which consist of `granularity_value` granules,
then these aggregates is used in `SELECT` queries for reducing amount of data to read from disk by skipping big blocks of data where `where` query can not be satisfied. then these aggregates are used in `SELECT` queries for reducing the amount of data to read from the disk by skipping big blocks of data where `where` query cannot be satisfied.
Example Example
@ -255,7 +255,7 @@ CREATE TABLE table_name
... ...
``` ```
Indices from the example can be used by ClickHouse to reduce amount of data to read from disk in following queries. Indices from the example can be used by ClickHouse to reduce the amount of data to read from disk in following queries.
```sql ```sql
SELECT count() FROM table WHERE s < 'z' SELECT count() FROM table WHERE s < 'z'
SELECT count() FROM table WHERE u64 * i32 == 10 AND u64 * length(s) >= 1234 SELECT count() FROM table WHERE u64 * i32 == 10 AND u64 * length(s) >= 1234
@ -263,7 +263,7 @@ SELECT count() FROM table WHERE u64 * i32 == 10 AND u64 * length(s) >= 1234
#### Available Types of Indices #### Available Types of Indices
* `minmax` Stores extremes of specified expression (if expression is `tuple`, then it stores extremes for each element of `tuple`), uses stored info for skipping blocks of data like primary key. * `minmax` Stores extremes of specified expression (if the expression is `tuple`, then it stores extremes for each element of `tuple`), uses stored info for skipping blocks of data like primary key.
```sql ```sql
INDEX sample_index (u64 * length(s)) TYPE minmax GRANULARITY 4 INDEX sample_index (u64 * length(s)) TYPE minmax GRANULARITY 4