<<| — Returns TRUE if A's bounding box is strictly below B's.
boolean <<|( geometry  A ,  geometry  B );
The <<| operator returns TRUE if the bounding box of geometry A is strictly below the bounding box of geometry B.
                 
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                 This operand will make use of any indexes that may be available on the geometries.  | 
            
SELECT tbl1.column1, tbl2.column1, tbl1.column2 <<| tbl2.column2 AS below
FROM
  ( VALUES
        (1, 'LINESTRING (0 0, 4 3)'::geometry)) AS tbl1,
  ( VALUES
        (2, 'LINESTRING (1 4, 1 7)'::geometry),
        (3, 'LINESTRING (6 1, 6 5)'::geometry),
        (4, 'LINESTRING (2 3, 5 6)'::geometry)) AS tbl2;
 column1 | column1 | below
---------+---------+-------
           1 |       2 | t
           1 |       3 | f
           1 |       4 | f
(3 rows)