From 68a64fe0b1399cd34b57d9644492767cdaae77fa Mon Sep 17 00:00:00 2001
From: Alex Cheng <alex.cheng@sunallies.com>
Date: Thu, 29 Feb 2024 20:47:58 +0800
Subject: [PATCH] Update 2024-02-07-clickhouse-tricks.md

---
 _posts/2024-02-07-clickhouse-tricks.md | 44 ++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/_posts/2024-02-07-clickhouse-tricks.md b/_posts/2024-02-07-clickhouse-tricks.md
index da10232..d4b5981 100644
--- a/_posts/2024-02-07-clickhouse-tricks.md
+++ b/_posts/2024-02-07-clickhouse-tricks.md
@@ -5,3 +5,47 @@
 set param_name='Alex';
 select {name:String};
 ```
+
+## projection的使用
+ClickHouse里的projection（投影）是物化的，也就是说数据会复制存一份。
+Projection对于不同的排序的查询的效率提升很有帮助，特别是行数很大的表。因为如果有一个projection的`order by`的设定跟查询的`order by`一样，则可以直接读取projection而不用排序数据。
+
+在2亿行数据的大宽表`variant_simulate._joined_events`上做实验。
+
+按照`_Dimension1_T1`排序，查询语句为：
+
+```sql
+select _Dimension1_T1 from  _joined_events order by _Dimension1_T1 format Null
+```
+
+时间是4秒。
+```sql
+Query id: 056df638-72b4-486f-b18a-94507ef2ecf7
+
+Ok.
+
+0 rows in set. Elapsed: 4.218 sec. Peak memory: 3.38 GiB. Processed 200.00 million rows, 1.80 GB (47.42 million rows/s., 426.78 MB/s.)
+Peak memory usage: 1.70 GiB.
+```
+
+添加projection投影，命名为`_dimension1_t1_proj`，并物化它，再执行同一个查询。
+
+```sql
+alter table _joined_events
+add projection _dimension1_t1_proj (
+        select _Dimension1_T1
+        from _joined_events
+        order by _Dimension1_T1
+    );
+    
+alter table _joined_events materialize projection _dimension1_t1_proj;
+```
+查询及执行结果为：
+```sql
+select _Dimension1_T1 from  _joined_events order by _Dimension1_T1 format Null
+
+0 rows in set. Elapsed: 1.874 sec. Peak memory: 3.38 GiB. Processed 200.00 million rows, 1.80 GB (106.73 million rows/s., 960.61 MB/s.)
+Peak memory usage: 14.95 MiB.
+```
+时间是1.8秒。快了2倍不止。
+