### Motivation
TABLESAMPLE allows you to limit the number of rows from a table in the FROM clause.
Used for data detection, quick verification of the accuracy of SQL, table statistics collection.
### Grammar
```
[TABLET tids] TABLESAMPLE n [ROWS | PERCENT] [REPEATABLE seek]
```
Limit the number of rows read from the table in the FROM clause,
select a number of Tablets pseudo-randomly from the table according to the specified number of rows or percentages,
and specify the number of seeds in REPEATABLE to return the selected samples again.
In addition, can also manually specify the TableID,
Note that this can only be used for OLAP tables.
### Example
Q1:
```
SELECT * FROM t1 TABLET(10001,10002) limit 1000;
```
explain:
```
partitions=1/1, tablets=2/12, tabletList=10001,10002
```
Select the specified tabletID of the t1.
Q2:
```
SELECT * FROM t1 TABLESAMPLE(1000000 ROWS) REPEATABLE 1 limit 1000;
```
explain:
```
partitions=1/1, tablets=3/12, tabletList=10001,10002,10003
```
Q3:
```
SELECT * FROM t1 TABLESAMPLE(1000000 ROWS) REPEATABLE 2 limit 1000;
```
explain:
```
partitions=1/1, tablets=3/12, tabletList=10002,10003,10004
```
Pseudo-randomly sample 1000 rows in t1.
Note that several Tablets are actually selected according to the statistics of the table,
and the total number of selected Tablet rows may be greater than 1000,
so if you want to explicitly return 1000 rows, you need to add Limit.
### Design
First, determine how many rows to sample from each partition according to the number of partitions.
Then determine the number of Tablets to be selected for each partition according to the average number of rows of Tablet,
If seek is not specified, the specified number of Tablets are pseudo-randomly selected from each partition.
If seek is specified, it will be selected sequentially from the seek tablet of the partition.
And add the manually specified Tablet id to the selected Tablet.
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