Re: Correct query to check streaming replication lag

[Granthana Biswas <granthana@xxxxxxxx>]

Thanks a load Michael. This is really helpful.

Regards,

Granthana

On Tue, Jan 21, 2014 at 12:19 PM, Michael Paquier <michael.paquier@xxxxxxxxx> wrote:

On Tue, Jan 21, 2014 at 2:33 PM, Sameer Kumar <sameer.kumar@xxxxxxxxxx> 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