Update Warning Section For FastScan (pingcap#13906)

tiflash/use-fastscan.md

@@ -12,6 +12,41 @@ By default, TiFlash guarantees the precision of query results and data consisten
 
 Some OLAP scenarios allow for some tolerance to the accuracy of the query results. In these cases, if you need higher query performance, you can enable the FastScan feature at the session or global level. You can choose whether to enable the FastScan feature by configuring the variable `tiflash_fastscan`.
 
+## Restrictions
+
+When the FastScan feature is enabled, your query results might include old data of a table. This means that you might get multiple historical versions of data with the same primary key or data that has been deleted.
+
+For example:
+
+```sql
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
+ALTER TABLE t1 SET TIFLASH REPLICA 1;
+INSERT INTO t1 VALUES(1,2);
+INSERT INTO t1 VALUES(10,20);
+UPDATE t1 SET b = 4 WHERE a = 1;
+DELETE FROM t1 WHERE a = 10;
+SET SESSION tidb_isolation_read_engines='tiflash';
+
+SELECT * FROM t1;
++------+------+
+| a    | b    |
++------+------+
+|    1 |    4 |
++------+------+
+
+SET SESSION tiflash_fastscan=ON;
+SELECT * FROM t1;
++------+------+
+| a    | b    |
++------+------+
+|    1 |    2 |
+|    1 |    4 |
+|   10 |   20 |
++------+------+
+```
+
+Although TiFlash can automatically initiate compaction of old data in the background, the old data will not be cleaned up physically until it has been compacted and its data versions are older than the GC safe point. After the physical cleaning, the cleaned old data will no longer be returned in FastScan mode. The timing of data compaction is automatically triggered by various factors. You can also manually trigger data compaction using the [`ALTER TABLE ... COMPACT`](/sql-statements/sql-statement-alter-table-compact.md) statement.
+
 ## Enable and disable FastScan
 
 By default, the variable is `tiflash_fastscan=OFF` at the session level and global level, that is, the FastScan feature is not enabled. You can view the variable information by using the following statement.
@@ -62,8 +97,8 @@ Data in the storage layer of TiFlash is stored in two layers: Delta layer and St
 By default, FastScan is not enabled, and the TableScan operator processes data in the following steps:
 
 1. Read data: create separate data streams in the Delta layer and Stable layer to read the respective data.
-2. Sort Merge: merge the data streams created in step 1. Then return the data after sorting in (handle, version) order.
+2. Sort Merge: merge the data streams created in step 1. Then return the data after sorting in the order of (primary key column, timestamp column).
 3. Range Filter: according to the data range, filter the data generated in step 2, and then return the data.
-4. MVCC + Column Filter: filter the data generated in step 3 through MVCC and filter out unneeded columns, and then return the data.
+4. MVCC + Column Filter: filter the data generated in step 3 through MVCC (that is, filtering the data version according to the primary key column and the timestamp column) and through columns (that is, filtering out unneeded columns), and then return the data.
 
 FastScan gains faster query speed by sacrificing some data consistency. Step 2 and the MVCC part in step 4 in the normal scan process are omitted in FastScan, thus improving query performance.