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# 如何确认查询是否使用了 PROJECTION？

> 了解如何通过样本数据测试并使用 EXPLAIN，检查 ClickHouse 查询是否使用了 PROJECTION。

<div id="question">
  ## 问题
</div>

如何判断是否使用了PROJECTION？

<div id="answer">
  ## 解答
</div>

1. 创建一个示例数据库

```
CREATE database db1;
```

2. 创建一个示例表，并将 column1 用作主键

```
CREATE table db1.table1_projections
(
 column1 Int32,
 column2 Int32
)
engine = MergeTree()
order by column1;
```

3. 添加PROJECTION `for_column2`，将 column2 用作主键

```
ALTER table db1.table1_projections add projection for_column2
(
  select * 
  order by column2
);
```

4. 插入测试数据

\*这会向 column1 和 column2 插入 100000 行随机数

```
INSERT INTO db1.table1_projections
select 
 floor(randNormal(50, 5)) as column1,
 floor(randUniform(1, 100)) as column2
from numbers(100000);
```

5. 检查样本数据

```
clickhouse-cloud :) SELECT * from db1.table1_projections limit 5;

SELECT *
FROM db1.table1_projections
LIMIT 5

Query id: d6940799-b507-4a5e-9843-df55ebe818ab

┌─column1─┬─column2─┐
│      28 │      41 │
│      29 │      12 │
│      30 │      73 │
│      30 │      75 │
│      30 │      70 │
└─────────┴─────────┘
```

6. 测试其是否使用了包含 column1 的原始表：

```
clickhouse-cloud :) explain indexes = 1 
                    SELECT count() from db1.table1_projections where column1 > 50;

EXPLAIN indexes = 1
SELECT count()
FROM db1.table1_projections
WHERE column1 > 50

Query id: e04d5236-1a05-4f1f-9502-7e41986beb44

┌─explain────────────────────────────────────────────┐
│ Expression ((Projection + Before ORDER BY))        │
│   Aggregating                                      │
│     Expression (Before GROUP BY)                   │
│       Filter (WHERE)                               │
│         ReadFromMergeTree (db1.table1_projections) │
│         Indexes:                                   │
│           PrimaryKey                               │
│             Condition: true                        │
│             Parts: 1/1                             │
│             Granules: 12/12                        │
└────────────────────────────────────────────────────┘
```

\*注意，这里读取的是 `db1.table1_projections`

7. 在 WHERE 子句中使用 column2，测试从PROJECTION中读取

```
clickhouse-cloud :) explain indexes = 1 
                    SELECT * from db1.table1_projections where column2 > 50;

EXPLAIN indexes = 1
SELECT *
FROM db1.table1_projections
WHERE column2 > 50

Query id: d2b20e01-93bf-4b60-a370-4aac7b454267

┌─explain─────────────────────────────────────┐
│ Expression ((Projection + Before ORDER BY)) │
│   Filter                                    │
│     ReadFromMergeTree (for_column2)         │
│     Indexes:                                │
│       PrimaryKey                            │
│         Keys:                               │
│           column2                           │
│         Condition: (column2 in [51, +Inf))  │
│         Parts: 1/1                          │
│         Granules: 6/12                      │
└─────────────────────────────────────────────┘
```

\*请注意，现在使用的是 `for_column2` PROJECTION。

**更多信息**

PROJECTION：
[https://clickhouse.com/docs/sql-reference/statements/alter/projection](https://clickhouse.com/docs/sql-reference/statements/alter/projection)

`numbers` 表函数： [https://clickhouse.com/docs/sql-reference/table-functions/numbers](https://clickhouse.com/docs/sql-reference/table-functions/numbers)s

关于生成随机数据的博客：
[https://clickhouse.com/blog/generating-random-test-distribution-data-for-clickhouse](https://clickhouse.com/blog/generating-random-test-distribution-data-for-clickhouse)