Add docs for tablesample system_rows()

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+
+
+
+ tsm_system_rows
+
+ 
+  tsm_system_rows
+ 
+
+ 
+  The tsm_system_rows module provides the tablesample method
+  SYSTEM_ROWS, which can be used inside the
+  TABLESAMPLE clause of a SELECT.
+ 
+
+ 
+  This tablesample method uses a linear probing algorithm to read sample
+  of a table and uses actual number of rows as limit (unlike the
+  SYSTEM tablesample method which limits by percentage
+  of a table).
+ 
+
+ 
+  Examples
+
+  
+   Here is an example of selecting sample of a table with
+   SYSTEM_ROWS. First install the extension:
+  
+
+
+CREATE EXTENSION tsm_system_rows;
+
+
+  
+   Then you can use it in SELECT command same way as other
+   tablesample methods:
+
+
+SELECT * FROM my_table TABLESAMPLE SYSTEM_ROWS(100);
+
+  
+
+  
+   The above command will return a sample of 100 rows from the table my_table
+   (less if the table does not have 100 visible rows).
+  
+ 
+
+