PMM-14411 - Enhance pg_custom_replication_wal query for accurate WAL …

[theTibi]

…metrics

Updated the pg_custom_replication_wal query to conditionally return the correct WAL metrics based on the database recovery state. Added logic to differentiate between received and current WAL LSNs, and adjusted lag calculation accordingly. Set the 'master' flag to true for this query to ensure it runs on the primary database instance.

[theTibi]

@kaminenibhargav can you have a look on this. Thanks.

[kaminenibhargav]

Hi Team - for some PG versions, we observe below error :

Oct 22 23:04:22 ip-10-25-2-73 pmm-agent[1574337]: time="2025-10-22T23:04:22.222-04:00" level=info msg="ts=2025-10-23T03:04:22.222Z caller=namespace.go:241 level=info err=\"Unexpected error parsing column: pg_custom_replication_wal replayed_lsn [52 56 68 67 47 56 55 70 70 70 55 54 48]\\n\"" agentID=c6398591-a78b-4e07-9a1f-3592432c40bf component=agent-process type=postgres_exporter

I think it's good to use pg_wal_lsn_diff to get the lag diff. Please check the below query

pg_custom_replication_wal:
  master: true
  query: |
    SELECT
      CASE WHEN pg_is_in_recovery() THEN 'replica' ELSE 'primary' END AS node_type,
      CASE WHEN pg_is_in_recovery() THEN pg_wal_lsn_diff(pg_last_wal_receive_lsn(), '0/0')::bigint ELSE NULL END AS received_lsn,
      CASE WHEN pg_is_in_recovery() THEN pg_wal_lsn_diff(pg_last_wal_replay_lsn(), '0/0')::bigint ELSE NULL END AS replayed_lsn,
      CASE WHEN pg_is_in_recovery() THEN NULL ELSE pg_wal_lsn_diff(pg_current_wal_lsn(), '0/0')::bigint END AS current_lsn,
      CASE
        WHEN pg_is_in_recovery() THEN pg_wal_lsn_diff(pg_last_wal_receive_lsn(), pg_last_wal_replay_lsn())
        ELSE NULL
      END AS lag_bytes;
   metrics:
    - node_type:
        usage: "LABEL"
        description: "Type of node (primary or replica)."
    - received_lsn:
        usage: "GAUGE"
        description: "Last WAL location received by the standby server (replica only) in bytes."
    - replayed_lsn:
        usage: "GAUGE"
        description: "Last WAL location replayed by the standby server (replica only) in bytes."
    - current_lsn:
        usage: "GAUGE"
        description: "Current WAL location on the primary server (primary only) in bytes."
    - lag_bytes:
        usage: "GAUGE"
        description: "Current WAL replication lag in bytes (replica only)."

pg_wal_lsn_diff(lsn1, lsn2) → returns a difference in bytes