pglockanalyze examines queries from the Postgresql's "pg_locks" system catalog and produces some summary output about which backend processes are blocked on which others.
This was really a one-off written to deal with a specific problem, but may be helpful for others in the future.
Specifically, the Postgres wiki suggests using this query to get a "flat view of blocking [processes]":
SELECT
waiting.locktype AS waiting_locktype,
waiting.relation::regclass AS waiting_table,
waiting_stm.query AS waiting_query,
waiting.mode AS waiting_mode,
waiting.pid AS waiting_pid,
other.locktype AS other_locktype,
other.relation::regclass AS other_table,
other_stm.query AS other_query,
other.mode AS other_mode,
other.pid AS other_pid,
other.GRANTED AS other_granted
FROM
pg_catalog.pg_locks AS waiting
JOIN
pg_catalog.pg_stat_activity AS waiting_stm
ON (
waiting_stm.pid = waiting.pid
)
JOIN
pg_catalog.pg_locks AS other
ON (
(
waiting."database" = other."database"
AND waiting.relation = other.relation
)
OR waiting.transactionid = other.transactionid
)
JOIN
pg_catalog.pg_stat_activity AS other_stm
ON (
other_stm.pid = other.pid
)
WHERE
NOT waiting.GRANTED
AND
waiting.pid <> other.pid
Each row in the pg_locks view represents a Postgresql backend process either holding or attempting to take a lock. Processes holding or attempting to take multiple locks may appear multiple times in the view.
The query above essentially selects all pairs of rows from "pg_locks" that are operating on the same lock, and then joins that information with the "pg_activity" information for each backend. In principle, this can show you when one process is blocking another, but interpretation is complicated by the fact that this query reports rows where neither process holds the lock (indicating they're both contending for the lock, but not necessarily blocking each other). It also reports rows for lock types that don't conflict. For example, the pids in a given row may both be attempting to take a shared (non-exclusive) lock on a table, in which case it's not a problem that they're both contending for it.
pglockanalyze is a Node.js program. You install it with:
$ npm install -g pglockanalyze
pglockanalyze takes on stdin the textual output you'd get by running this query using psql(1), and produces on stdout a report like this:
$ psql -c "..." > query.out
$ pglockanalyze < query.out
...
pid 93744: query: SELECT *, '488f0950-bf92-405a-8b77-ae700accca48' AS req_id FROM manta WHERE _key=$1
wants AccessShareLock: may contend with 1 pids wanting AccessExclusiveLock (e.g., 42367)
wants AccessShareLock: may contend with 983 pids wanting AccessShareLock (e.g., 93618)
wants AccessShareLock: may contend with 9 pids wanting RowShareLock (e.g., 93737)
pid 93759: query: SELECT *, '8dce62c2-4f42-4c70-87be-d62b1c1f84f4' AS req_id FROM manta WHERE _key=$1 FOR UPDATE
wants RowShareLock: may contend with 1 pids wanting AccessExclusiveLock (e.g., 42367)
wants RowShareLock: may contend with 984 pids wanting AccessShareLock (e.g., 93618)
wants RowShareLock: may contend with 8 pids wanting RowShareLock (e.g., 93737)
pid 91544: query: autovacuum: VACUUM ANALYZE public.manta (to prevent wraparound)
holds ShareUpdateExclusiveLock: may contend with 1 pids wanting AccessExclusiveLock (e.g., 42367)
holds ShareUpdateExclusiveLock: may contend with 984 pids wanting AccessShareLock (e.g., 93618)
holds ShareUpdateExclusiveLock: may contend with 9 pids wanting RowShareLock (e.g., 93737)
pid 42367: query: DROP TRIGGER IF EXISTS count_directories ON manta
holds AccessShareLock: may contend with 984 pids wanting AccessShareLock (e.g., 93618)
holds AccessShareLock: may contend with 9 pids wanting RowShareLock (e.g., 93737)
wants AccessExclusiveLock: may contend with 984 pids wanting AccessShareLock (e.g., 93618)
wants AccessExclusiveLock: may contend with 9 pids wanting RowShareLock (e.g., 93737)
In order to make sense of this, you're really going to have to read and understand the Postgresql documentation on explicit locking.
There's also a "json" mode that converts the rows to json. You can then query them with tools like json(1):
$ pglockanalyze json < query.out > locks.json
- It would be easier (both for users and for the implementation of this program) if the input were actually just the contents of the "pg_locks" and "pg_activity" views, rather than pairs of pids that might be related.
- Relatedly, instead of looking at pairs of processes blocking each other, it would be useful to pool together all information about a single lock.