Redshift is a one of the most popular data warehousing solution, thousands of companies running millions of ETL jobs everyday. The problem with MPP systems is troubleshooting why the jobs are hung, which are the queries blocking others.
To troubleshoot problems like this could be a real nightmare if you are new to Redshift, in this artical I have tried to aggregate the tables and queries you should alwas keep handy if you work with redshift on daily basis of planning to start using
First of all lets familiarize our self with some of the tables needed to troubleshoot a problem.
STV_RECENTS - This table holds information about currently active and recently run queries against a database
select user_name, db_name, pid, query from stv_recents where status = 'Running';
status = 'Running' gives all the queries whose execution have not completed. it includes the queries which are currently executing and the queries currently waiting in the execution queue.
STV_INFLIGHT - Check the
stv_inflight table, To find which queries are currently in progress.
select userid, query, pid, starttime, left(text, 50) as text from stv_inflight;
STV_LOCKS - Amazon Redshift locks tables to prevent two users from updating the same table at the same time, STV_LOCKS can be used to view any current updates on tables in the database, need superuser to view
select table_id, last_update, last_commit, lock_owner, lock_owner_pid, lock_status from stv_locks order by last_update asc;
STL_TR_CONFLICT - A transaction conflict occurs when two or more users are querying and modifying data rows from tables such that their transactions cannot be serialized. Every time a transaction conflict occurs, Amazon Redshift writes a log about the aborted transaction to the STL_TR_CONFLICT table.
select * from stl_tr_conflict where table_id=<Table Id from STV_LOCKS> order by xact_start_ts;
SVV_TRANSACTIONS - Redshift uses this table to records information about transactions that currently hold locks on tables in the database
select * from svv_transactions;
Queries waiting in queue
If the query is runing for more then expected the fisrt this you would like to do is figure out if the query actully executing or laying in the queue waiting for its turn.
To find out queries that are not truly “in flight” i.e waiting in the queue of blocked by some other query
select * from stv_recents where status<>'Done' and pid not in (select pid from stv_inflight)
Long Running Queries
In case you are curious to know who else is delayed or running for long time, this query can help you find out list of all the queries running longer then 30 mints.
select pid, trim(user_name) AS user_name, starttime, query, DATEDIFF(minutes, starttime, getdate()) as delay_in_mints, status from stv_recents where status='Running' and DATEDIFF(minutes, starttime, getdate()) > 30 order by starttime;
To find out the cause you must verify the locks this query can be used to find out what are the queries which have been granted the lock for the resources and what are the queries blocked by it or waiting for the same lock
select b.*, w.pid as blocked_pid, w.txn_owner as blocked_owner, DATEDIFF(minutes, b.txn_start, getdate()) as blocked_for_mints from SVV_TRANSACTIONS b inner join SVV_TRANSACTIONS w on b.txn_db = w.txn_db and b.relation = w.relation where b.granted='t' and w.granted = 'f' and DATEDIFF(minutes, b.txn_start, getdate()) > 5 order by txn_start, b.pid;
granted = t means lock has been granted
granted = f means lock is pending
Redshift documentation recommens using STV_LOCKS table to identify locks, this table works well untill you hit a real deadlock, PG_LOCKS could be the real life saving table that should be looked into.
select current_time, c.relname, l.database, l.transaction, l.pid, a.usename, l.mode, l.granted from pg_locks l join pg_catalog.pg_class c ON c.oid = l.relation join pg_catalog.pg_stat_activity a ON a.procpid = l.pid where l.pid <> pg_backend_pid();
How to cancel a query?
cancel` can be used to Kill a query with the query pid and an optional message which will be returned to the issuer of the query and logged. PG_CANCEL_BACKEND is functionally equivalent to the CANCEL command.
cancel <pid> 'Long-running query';
If the query that you canceled is associated with a transaction, use the ABORT or ROLLBACK. command to cancel the transaction and discard any changes made to the data:
PG_TERMINATE_BACKEND can be used to Terminates a session.
Unless you are signed on as a superuser, you can cancel only your own queries/session. A superuser can cancel all queries/session.
Some more Tables to for more informations
SVL_QLOG - Redshift also stores the past few days of queries in
svl_qlog if you need to go back further
select userid, query, pid, starttime, endtime, elapsed, left("substring", 50) as text from svl_qlog limit 10;
STL_QUERYTEXT All of the above tables only store the first 200 characters of each query. The full query is stored in chunks in
stl_querytext. Join this table in by
query, and sort by
sequence to get each 200 character chunk in order
select query, starttime, text, "sequence" from stl_query join stl_querytext using (query) order by query,sequence limit 5;
List of queries currently in-flight with user details
select a.userid, cast(u.usename as varchar(100)), a.query, a.label, a.pid, a.starttime, DATEDIFF(minutes, a.starttime, getdate()) as delay_in_mints, b.query as querytext from stv_inflight a, stv_recents b, pg_user u where a.pid = b.pid and a.userid = u.usesysid;