闪回查询

背景信息

闪回查询可以查询过去某个时间点表的某个snapshot数据，这一特性可用于查看和逻辑重建意外删除或更改的受损数据。闪回查询基于MVCC多版本机制，通过检索查询旧版本，获取指定老版本数据。

前提条件

undo_retention_time参数用于设置undo旧版本的保留时间。

语法

{[ ONLY ] table_name [ * ] [ partition_clause ] [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
[ TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed ) ] ]
[TIMECAPSULE { TIMESTAMP | CSN } expression ]
|( select ) [ AS ] alias [ ( column_alias [, ...] ) ]
|with_query_name [ [ AS ] alias [ ( column_alias [, ...] ) ] ]
|function_name ( [ argument [, ...] ] ) [ AS ] alias [ ( column_alias [, ...] | column_definition [, ...] ) ]
|function_name ( [ argument [, ...] ] ) AS ( column_definition [, ...] )
|from_item [ NATURAL ] join_type from_item [ ON join_condition | USING ( join_column [, ...] ) ]}

参数说明

• TIMESTAMP
  • 指要查询某个表在TIMESTAMP这个时间点上的数据，TIMESTAMP指一个具体的历史时间。

使用示例

• 示例：

  gaussdb=# drop TABLE IF EXISTS "public".flashtest;
  NOTICE:  table "flashtest" does not exist, skipping
  DROP TABLE
  --创建表flashtest
  gaussdb=# CREATE TABLE "public".flashtest (col1 INT,col2 TEXT) with(storage_type=ustore);
  NOTICE:  The 'DISTRIBUTE BY' clause is not specified. Using 'col1' as the distribution column by default.
  HINT:  Please use 'DISTRIBUTE BY' clause to specify suitable data distribution column.
  CREATE TABLE
  --查询csn
  gaussdb=# select int8in(xidout(next_csn)) from gs_get_next_xid_csn();
    int8in  
  ----------
   79351682
   79351682
   79351682
   79351682
   79351682
   79351682
  (6 rows)
  --查询当前时间戳
  gaussdb=# select now();
                now              
  -------------------------------
   2023-09-13 19:35:26.011986+08
  (1 row)
  --插入数据
  gaussdb=# INSERT INTO flashtest VALUES(1,'INSERT1'),(2,'INSERT2'),(3,'INSERT3'),(4,'INSERT4'),(5,'INSERT5'),(6,'INSERT6');
  INSERT 0 6
  gaussdb=# SELECT * FROM flashtest;
   col1 |  col2   
  ------+---------
      3 | INSERT3
      1 | INSERT1
      2 | INSERT2
      4 | INSERT4
      5 | INSERT5
      6 | INSERT6
  (6 rows)
  --闪回查询某个csn处的表
  gaussdb=# SELECT * FROM flashtest TIMECAPSULE CSN 79351682;
   col1 | col2 
  ------+------
  (0 rows)
  gaussdb=# SELECT * FROM flashtest;
   col1 |  col2   
  ------+---------
      1 | INSERT1
      2 | INSERT2
      4 | INSERT4
      5 | INSERT5
      3 | INSERT3
      6 | INSERT6
  (6 rows)
  --闪回查询某个时间戳处的表
  gaussdb=# SELECT * FROM flashtest TIMECAPSULE TIMESTAMP '2023-09-13 19:35:26.011986';
   col1 | col2 
  ------+------
  (0 rows)
  gaussdb=# SELECT * FROM flashtest;
   col1 |  col2   
  ------+---------
      1 | INSERT1
      2 | INSERT2
      4 | INSERT4
      5 | INSERT5
      3 | INSERT3
      6 | INSERT6
  (6 rows)
  --闪回查询某个时间戳处的表
  gaussdb=# SELECT * FROM flashtest TIMECAPSULE TIMESTAMP to_timestamp ('2023-09-13 19:35:26.011986', 'YYYY-MM-DD HH24:MI:SS.FF');
   col1 | col2 
  ------+------
  (0 rows)
  --闪回查询某个csn处的表，并对表进行重命名
  gaussdb=# SELECT * FROM flashtest AS ft TIMECAPSULE CSN 79351682;
   col1 | col2 
  ------+------
  (0 rows)
  gaussdb=# drop TABLE IF EXISTS "public".flashtest;
  DROP TABLE