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do not modify column type when there is statistic

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This commit is contained in:
Han Fei 2023-10-11 16:57:40 +02:00
parent fcb19b4f77
commit 253b8efa58
4 changed files with 39 additions and 4 deletions
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19
docs/en/engines/table-engines/mergetree-family/mergetree.md
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@ -1358,16 +1358,27 @@ In this sample configuration:
## Column Statistics (Experimental) {#column-statistics}
The statistic declaration is in the columns section of the `CREATE` query.
The statistic declaration is in the columns section of the `CREATE` query for tables from the `*MergeTree*` Family when we enable `set allow_experimental_statistic = 1`.
``` sql
STATISTIC(type)
CREATE TABLE example_table
(
a Int64 STATISTIC(tdigest),
b Float64
)
ENGINE = MergeTree
ORDER BY a
```
For tables from the `*MergeTree` family, statistics can be specified.
We can also manipulate statistics with `ATLER` statements.
```sql
ATLER TABLE example_table ADD STATISTIC b TYPE tdigest;
ATLER TABLE example_table DROP STATISTIC a TYPE tdigest;
```
These lightweight statistics aggregate information about distribution of values in columns.
They can be used for query optimization (At current time they are used for moving expressions to PREWHERE).
They can be used for query optimization when we enable `set allow_statistic_optimize = 1`.
#### Available Types of Column Statistics {#available-types-of-column-statistics}

8
docs/en/operations/settings/settings.md
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@ -4726,3 +4726,11 @@ a Tuple(
l Nullable(String)
)
```
## allow_experimental_statistic {#allow_experimental_statistic}
Allows defining columns with [statistics](../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-creating-a-table) and [manipulate statistics](../../engines/table-engines/mergetree-family/mergetree.md#column-statistics).
## allow_statistic_optimize {#allow_statistic_optimize}
Allows using statistic to optimize the order of [prewhere conditions](../../sql-reference/statements/select/prewhere.md).

11
src/Storages/MergeTree/MergeTreeData.cpp
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@ -3312,6 +3312,17 @@ void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, Context
{
columns_to_check_conversion.push_back(
new_metadata.getColumns().getPhysical(command.column_name));
const auto & old_column = old_metadata.getColumns().get(command.column_name);
if (old_column.stat)
{
const auto & new_column = new_metadata.getColumns().get(command.column_name);
if (!old_column.type->equals(*new_column.type))
throw Exception(ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN,
"ALTER types of column {} with statistic is not not safe "
"because it can change the representation of statistic",
backQuoteIfNeed(command.column_name));
}
}
}
}

5
tests/queries/0_stateless/02864_statistic_exception.sql
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@ -45,4 +45,9 @@ ALTER TABLE t1 DROP STATISTIC a TYPE tdigest; -- { serverError ILLEGAL_STATISTIC
ALTER TABLE t1 CLEAR STATISTIC a TYPE tdigest; -- { serverError ILLEGAL_STATISTIC }
ALTER TABLE t1 MATERIALIZE STATISTIC b TYPE tdigest; -- { serverError ILLEGAL_STATISTIC }
ALTER TABLE t1 ADD STATISTIC a TYPE tdigest;
ALTER TABLE t1 ADD STATISTIC b TYPE tdigest;
ALTER TABLE t1 MODIFY COLUMN a Float64 TTL now() + INTERVAL 1 MONTH;
ALTER TABLE t1 MODIFY COLUMN a Int64; -- { serverError ALTER_OF_COLUMN_IS_FORBIDDEN }
DROP TABLE t1;