Merge pull request #7521 from soyeric128/system-functions

docs: system functions

Author: BohuTANG

docs/doc/30-reference/20-functions/111-system-functions/_category_.json

@@ -0,0 +1,7 @@
+{
+  "label": "System Functions",
+  "link": {
+    "type": "generated-index",
+    "slug": "/reference/functions/system-functions"
+  }
+}

docs/doc/30-reference/20-functions/111-system-functions/clustering_information.md

@@ -0,0 +1,28 @@
+---
+title: CLUSTERING_INFORMATION
+---
+
+Returns clustering information of a table.
+
+## Syntax
+
+```sql
+CLUSTERING_INFORMATION(‘<database_name>’, ‘<table_name>’)
+```
+
+## Examples
+
+```sql
+CREATE TABLE mytable(a int, b int) CLUSTER BY(a+1);
+
+INSERT INTO mytable VALUES(1,1),(3,3);
+INSERT INTO mytable VALUES(2,2),(5,5);
+INSERT INTO mytable VALUES(4,4);
+
+SELECT * FROM CLUSTERING_INFORMATION(‘default‘,’mytable‘);
+
+---
+| cluster_by_keys | total_block_count | total_constant_block_count | average_overlaps | average_depth | block_depth_histogram |
+|-----------------|-------------------|----------------------------|------------------|---------------|-----------------------|
+| ((a + 1))       | 3                 | 1                          | 1.3333           | 2.0           | {"00002":3}           |
+```

docs/doc/30-reference/20-functions/111-system-functions/fuse_segment.md

@@ -0,0 +1,39 @@
+---
+title: FUSE_SEGMENT
+---
+
+Returns the segment information of a snapshot of table.
+
+## Syntax
+
+```sql
+FUSE_SEGMENT('<database_name>', '<table_name>','<snapshot_id>')
+```
+
+## Examples
+
+```sql
+CREATE TABLE mytable(c int);
+INSERT INTO mytable values(1);
+INSERT INTO mytable values(2); 
+
+-- Obtain a snapshot ID
+SELECT snapshot_id FROM FUSE_SNAPSHOT('default', 'mytable') limit 1;
+
+---
++----------------------------------+
+| snapshot_id                      |
++----------------------------------+
+| 82c572947efa476892bd7c0635158ba2 |
++----------------------------------+
+
+SELECT * FROM FUSE_SEGMENT('default', 'mytable', '82c572947efa476892bd7c0635158ba2');
+
+---
++----------------------------------------------------+----------------+-------------+-----------+--------------------+------------------+
+| file_location                                      | format_version | block_count | row_count | bytes_uncompressed | bytes_compressed |
++----------------------------------------------------+----------------+-------------+-----------+--------------------+------------------+
+| 1/319/_sg/d35fe7bf99584301b22e8f6a8a9c97f9_v1.json |              1 |           1 |         1 |                  4 |              184 |
+| 1/319/_sg/c261059d47c840e1b749222dabb4b2bb_v1.json |              1 |           1 |         1 |                  4 |              184 |
++----------------------------------------------------+----------------+-------------+-----------+--------------------+------------------+
+```

docs/doc/30-reference/20-functions/111-system-functions/fuse_snapshot.md

@@ -0,0 +1,30 @@
+---
+title: FUSE_SNAPSHOT
+---
+
+Returns the snapshot information of a table for querying previous versions of your data. For more information, see [AT](../../30-sql/20-query-syntax/dml-at.md).
+
+## Syntax
+
+```sql
+FUSE_SNAPSHOT('<database_name>', '<table_name>')
+```
+
+## Examples
+
+```sql
+CREATE TABLE mytable(a int, b int) CLUSTER BY(a+1);
+
+INSERT INTO mytable VALUES(1,1),(3,3);
+INSERT INTO mytable VALUES(2,2),(5,5);
+INSERT INTO mytable VALUES(4,4);
+
+SELECT * FROM FUSE_SNAPSHOT(‘default‘,’mytable‘);
+
+---
+| snapshot_id                      | snapshot_location                                          | format_version | previous_snapshot_id             | segment_count | block_count | row_count | bytes_uncompressed | bytes_compressed | timestamp                  |
+|----------------------------------|------------------------------------------------------------|----------------|----------------------------------|---------------|-------------|-----------|--------------------|------------------|----------------------------|
+| dd98266f968f4817b470255592d04fda | 670655/670675/_ss/dd98266f968f4817b470255592d04fda_v1.json | 1              | \N                               | 1             | 1           | 2         | 16                 | 290              | 2022-09-07 01:58:55.204997 |
+| 2f2d004ff6f842cdb25f5631b2bbb703 | 670655/670675/_ss/2f2d004ff6f842cdb25f5631b2bbb703_v1.json | 1              | dd98266f968f4817b470255592d04fda | 2             | 2           | 4         | 32                 | 580              | 2022-09-07 01:59:09.742999 |
+| 0aa6dfd5d5364bde80f21161ba48c96e | 670655/670675/_ss/0aa6dfd5d5364bde80f21161ba48c96e_v1.json | 1              | 2f2d004ff6f842cdb25f5631b2bbb703 | 3             | 3           | 5         | 40                 | 862              | 2022-09-07 01:59:16.858454 |
+```

docs/doc/30-reference/30-sql/20-query-syntax/dml-at.md

@@ -2,7 +2,7 @@
 title: AT
 ---
 
-The SELECT statement can include an AT clause that allows you to  query previous versions of your data by a specific snapshot ID or timestamp.
+The SELECT statement can include an AT clause that allows you to query previous versions of your data by a specific snapshot ID or timestamp.
 
 Databend automatically creates snapshots when data updates occur, so a snapshot can be considered as a view of your data at a time point in the past. You can access a snapshot by the snapshot ID or the timestamp at which the snapshot was created. For how to obtain the snapshot ID and timestamp, see [Obtaining Snapshot ID and Timestamp](#obtaining-snapshot-id-and-timestamp).
 
@@ -26,12 +26,7 @@ SELECT snapshot_id,
 FROM   fuse_snapshot('<database_name>', '<table_name>'); 
 ```
 
-You can find more information related to the snapshots by executing the following statement:
-
-```sql
-SELECT * 
-FROM   Fuse_snapshot('<database_name>', '<table_name>'); 
-```
+For more information about the FUSE_SNAPSHOT function,see [FUSE_SNAPSHOT](../../20-functions/111-system-functions/fuse_snapshot.md).
 
 ## Examples