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Update column.md

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docs/en/sql-reference/statements/alter/column.md
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@ -197,12 +197,13 @@ ALTER TABLE table_with_ttl MODIFY COLUMN column_ttl REMOVE TTL;
## MATERIALIZE COLUMN {#materialize-column}
Materializes the column in the parts where the column is missing. This is useful in case of creating a new column with complicated `DEFAULT` or `MATERIALIZED` expression. Calculation of the column directly on `SELECT` query can cause bigger request execution time, so it is reasonable to use `MATERIALIZE COLUMN` for such columns. To perform same manipulation for existing column, use `FINAL` modifier.
Materializes or updates a column with an expression for a default value (`DEFAULT` or `MATERIALIZED`).
It is used if it is necessary to add or update a column with a complicated expression, because evaluating such an expression directly on `SELECT` executing turns out to be expensive.
Syntax:
```sql
ALTER TABLE table MATERIALIZE COLUMN col [FINAL];
ALTER TABLE table MATERIALIZE COLUMN col;
```
**Example**
@ -211,20 +212,34 @@ ALTER TABLE table MATERIALIZE COLUMN col [FINAL];
DROP TABLE IF EXISTS tmp;
SET mutations_sync = 2;
CREATE TABLE tmp (x Int64) ENGINE = MergeTree() ORDER BY tuple() PARTITION BY tuple();
INSERT INTO tmp SELECT * FROM system.numbers LIMIT 10;
INSERT INTO tmp SELECT * FROM system.numbers LIMIT 5;
ALTER TABLE tmp ADD COLUMN s String MATERIALIZED toString(x);
ALTER TABLE tmp MATERIALIZE COLUMN s;
SELECT groupArray(x), groupArray(s) FROM (select x,s from tmp order by x);
┌─groupArray(x)─┬─groupArray(s)─────────┐
│ [0,1,2,3,4] │ ['0','1','2','3','4'] │
└───────────────┴───────────────────────┘
ALTER TABLE tmp MODIFY COLUMN s String MATERIALIZED toString(round(100/x));
INSERT INTO tmp SELECT * FROM system.numbers LIMIT 5,5;
SELECT groupArray(x), groupArray(s) FROM tmp;
```
**Result:**
┌─groupArray(x)─────────┬─groupArray(s)──────────────────────────────────┐
│ [0,1,2,3,4,5,6,7,8,9] │ ['0','1','2','3','4','20','17','14','12','11'] │
└───────────────────────┴────────────────────────────────────────────────┘
```sql
┌─groupArray(x)─────────┬─groupArray(s)─────────────────────────────┐
│ [0,1,2,3,4,5,6,7,8,9] │ ['0','1','2','3','4','5','6','7','8','9'] │
└───────────────────────┴───────────────────────────────────────────┘
ALTER TABLE tmp MATERIALIZE COLUMN s;
SELECT groupArray(x), groupArray(s) FROM tmp;
┌─groupArray(x)─────────┬─groupArray(s)─────────────────────────────────────────┐
│ [0,1,2,3,4,5,6,7,8,9] │ ['inf','100','50','33','25','20','17','14','12','11'] │
└───────────────────────┴───────────────────────────────────────────────────────┘
```
**See Also**