SELECT * FROM pgperf.snapshot_pg_current_xlog ORDER BY 1;
 sid | location | insert_location 
-----+----------+-----------------
(0 rows)

SELECT pgperf.create_snapshot_pg_current_xlog(0);
 create_snapshot_pg_current_xlog 
---------------------------------
 t
(1 row)

SELECT pgperf.create_snapshot_pg_current_xlog(1);
 create_snapshot_pg_current_xlog 
---------------------------------
 t
(1 row)

WITH s0 AS (
SELECT * FROM pgperf.snapshot_pg_current_xlog WHERE sid = 0
), s1 AS (
SELECT * FROM pgperf.snapshot_pg_current_xlog WHERE sid = 1
)
SELECT s0.location < s1.location, s0.insert_location < s1.insert_location
  FROM s0, s1
 WHERE s0.sid + 1 = s1.sid;
 ?column? | ?column? 
----------+----------
 t        | t
(1 row)

SELECT count(*) FROM pgperf.snapshot_pg_current_xlog;
 count 
-------
     2
(1 row)

SELECT pgperf.delete_snapshot_pg_current_xlog(1);
 delete_snapshot_pg_current_xlog 
---------------------------------
 t
(1 row)

SELECT count(*) FROM pgperf.snapshot_pg_current_xlog;
 count 
-------
     1
(1 row)

SELECT pgperf.delete_snapshot_pg_current_xlog(0);
 delete_snapshot_pg_current_xlog 
---------------------------------
 t
(1 row)

SELECT count(*) FROM pgperf.snapshot_pg_current_xlog;
 count 
-------
     0
(1 row)