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Adds docs for ASSUME CONSTRAINT.

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johnnymatthews 2024-01-10 21:13:21 -04:00
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docs/en/sql-reference/statements/create/table.md
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@ -293,6 +293,8 @@ You can't combine both ways in one query.
Along with columns descriptions constraints could be defined:
### CONSTRAINT
``` sql
CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
@ -307,6 +309,30 @@ CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
Adding large amount of constraints can negatively affect performance of big `INSERT` queries.
### ASSUME
The `ASSUME` clause is used to define a `CONSTRAINT` on a table that is assumed to be true. This constraint can then be used by the optimizer to enhance the performance of SQL queries.
Take this example where `ASSUME CONSTRAINT` is used in the creation of the `users_a` table:
```sql
CREATE TABLE users_a (
uid Int16,
name String,
age Int16,
name_len UInt8 MATERIALIZED length(name),
CONSTRAINT c1 ASSUME length(name) = name_len
)
ENGINE=MergeTree
ORDER BY (name_len, name);
```
Here, `ASSUME CONSTRAINT` is used to assert that the `length(name)` function always equals the value of the `name_len` column. This means that whenever `length(name)` is called in a query, ClickHouse can replace it with `name_len`, which should be faster because it avoids calling the `length()` function.
Then, when executing the query `SELECT name FROM users_a WHERE length(name) < 5;`, ClickHouse can optimize it to `SELECT name FROM users_a WHERE name_len < 5`; because of the `ASSUME CONSTRAINT`. This can make the query run faster because it avoids calculating the length of `name` for each row.
`ASSUME CONSTRAINT` **does not enforce the constraint**, it merely informs the optimizer that the constraint holds true. If the constraint is not actually true, the results of the queries may be incorrect. Therefore, you should only use `ASSUME CONSTRAINT` if you are sure that the constraint is true.
## TTL Expression
Defines storage time for values. Can be specified only for MergeTree-family tables. For the detailed description, see [TTL for columns and tables](../../../engines/table-engines/mergetree-family/mergetree.md#table_engine-mergetree-ttl).