Re: Correct query to check streaming replication lag

On Tue, Jan 21, 2014 at 2:33 PM, Sameer Kumar <sameer(dot)kumar(at)ashnik(dot)com>
wrote:
>>
>>
>> We are already using the following query:
>>
>> SELECT CASE WHEN pg_last_xlog_receive_location(
>> ) = pg_last_xlog_replay_location() THEN 0 ELSE EXTRACT (EPOCH FROM now()
-
>> pg_last_xact_replay_timestamp()) END AS log_delay;
>>
> This is (delay) not the correct thing to monitor.
>
>> We cannot use pg_xlog_location_diff as we use postgresql 9.1.
>>
> You can still use the other two methods I mentioned.

FYI, here is an equivalent written in plpgsql easily findable by googling a
bit, making a pg_xlog_location_diff-like function usable even in 9.1 and
9.0 servers:
CREATE OR REPLACE FUNCTION pg_xlog_location_diff_sql( text, text)
RETURNS numeric
LANGUAGE plpgsql
AS
$function$
DECLARE
offset1 text;
offset2 text;
xlog1 text;
xlog2 text;
SQL text;
diff text;
BEGIN
/* Extract the Offset and xlog from input in
offset and xlog variables */

offset1=split_part($1,'/',2);
xlog1=split_part($1,'/',1);
offset2=split_part($2,'/',2);
xlog2=split_part($2,'/',1);

/* Prepare SQL query for calculation based on following formula
(FF000000 * xlog + offset) - (FF000000 * xlog + offset)
which gives value in hexadecimal. Since, hexadecimal calculation
is cumbersome
so convert into decimal and then calculate the difference */

SQL='SELECT (x'''||'FF000000'||'''::bigint * x'''||xlog1||'''::bigint
+ x'''||offset1||'''::bigint)'||'
-
(x'''||'FF000000'||'''::bigint * x'''||xlog2||'''::bigint
+ x'''||offset2||'''::bigint)';
EXECUTE SQL into diff;

/* Return the value in numeric by explicit casting */

RETURN diff::numeric;
END;
$function$;

Source:
http://vibhorkumar.wordpress.com/2013/02/18/pg_xlog_location_diff-function-for-postgreqsqlppas/
--
Michael