Technotes for future me



Using Regular Expressions

You can edit column using regular expressions by running regexp_replace()

UPDATE table SET field=regexp_replace(field, 'match pattern', 'replace string', 'g');

JSON Output

You can get Postgres to output JSON like this:

SELECT row_to_json(<name of key column>) FROM ...

Analyze Queries

EXPLAIN ANALYZE <sql statement>;

Inspect an Installation

List Postgres Clusters

Under Debian use the pg_wrapper command


List Postgres Settings


List Databases and Sizes

SELECT pg_database.datname, pg_size_pretty(pg_database_size(pg_database.datname)) AS size FROM pg_database;

Show Running Queries in Postgres

SELECT * FROM pg_stat_activity;

Show Blocking Locks

SELECT AS blocked_pid, a.usename AS blocked_user, AS blocking_pid, ka.usename AS blocking_user FROM pg_catalog.pg_locks bl JOIN pg_catalog.pg_stat_activity a ON = JOIN pg_catalog.pg_locks kl JOIN pg_catalog.pg_stat_activity ka ON = ON bl.transactionid = kl.transactionid AND != WHERE NOT bl.granted ;

Show Table Usage

If you want to know accesses or I/O per table or index you can use the pg_stat_*_tables and pg_statio_*_tables relations. For example:

SELECT * FROM pg_statio_user_tables;

to show the I/O caused by your relations. Or for the number of accesses and scan types and tuples fetched:

SELECT * FROM pg_stat_user_tables;


Changing Live Settings

ALTER SYSTEM SET <setting> TO <value>;
SELECT pg_reload_conf();

Kill Postgres Query

First find the query and it’s PID:

SELECT procpid, current_query FROM pg_stat_activity;

And then kill the PID on the Unix shell. Or use

SELECT pg_terminate_backend('12345');

Kill all Connections to a DB

The following was suggested here. Replace “TARGET_DB” with the name of the database whose connections should be killed.

SELECT pg_terminate_backend(pg_stat_activity.procpid) FROM pg_stat_activity WHERE pg_stat_activity.datname = 'TARGET_DB';

Checking Replication

Compared to MySQL checking for replication delay is rather hard. It is usually good to script this or use ready monitoring tools (e.g. Nagios Postgres check). Still it can be done manually by running this command on the master:

SELECT pg_current_xlog_location();

and those two commands on the slave:

SELECT pg_last_xlog_receive_location();
SELECT pg_last_xlog_replay_location()

The first query gives you the most recent xlog position on the master, while the other two queries give you the most recently received xlog and the replay position in this xlog on the slave. A Nagios check plugin could look like this:


# Checks master and slave xlog difference...
# Pass slave IP/host via $1

PSQL="psql -A -t "

# Get master status
master=$(echo "SELECT pg_current_xlog_location();" | $PSQL)

# Get slave receive location
slave=$(echo "select pg_last_xlog_replay_location();" | $PSQL -h$1)

master=$(echo "$master" | sed "s/\/.*//")
slave=$(echo "$slave" | sed "s/\/.*//")

master_dec=$(echo "ibase=16; $master" | bc)
slave_dec=$(echo "ibase=16; $slave" | bc)
diff=$(expr $master_dec - $slave_dec)

if [ "$diff" == "" ]; then
    echo "Failed to retrieve replication info!"
    exit 3

# Choose some good threshold here...
if [ $diff -gt 3 ]; then
if [ $diff -gt 5 ]; then

echo "Master at $master, Slave at $slave , difference: $diff"
exit $status

Postgres Backup Mode

To be able to copy Postgres files e.g. to a slave or a backup you need to put the server into backup mode.

SELECT pg_start_backup('label', true);
SELECT pg_stop_backup();

Read more: Postgres - Set Backup Mode

Pooling / Failover / LB

There are two connection pooling solutions for Postgres both providing read traffic load balancing and HA for read only slaves:

Additionally there is repmgr which manages and monitors replication and has automatic slave promotion on master failure.

Log Analysis

Further Reading



The must have reading for Postgres is for sure this book:\

Last updated on 31 Jan 2021
Published on 25 Dec 2019
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