PostgreSQL
General Tips
Optimize GIN indexes updates
While GIN Indexes make querying jsonb faster they are also expensive to update. Especially because a single change can
cause the update of multiple index entries. In order to keep the overhead on write and update statements low, postgres
per default enables the fastupdate
setting which defers the update of the index and instead gathers changes to execute
them all at once. This update happens when:
- the
gin_pending_list_limit
is reached (default 4MB) - the
gin_clean_pending_list
function is called - at the end of the autovacuum of the table
This can cause the query whose change eventually fills the gin_pending_list_limit
to be a lot slower than usual. If
this kind of behavior is observed it might make sense to consider to:
- reduce the
gin_pending_list_limit
-> more frequent, smaller flushes - increase the limit and do manual flushes outside of workload
- turn off
fastupdate
- let autovacuum run more often or manually call the clean operation
Autoanalyse & Autovacuum settings
Postgres has a built-in mechanism to keep the statistics up to date and to clean up dead tuples, called Automatic Vacuuming.
In most cases, it might be necessary to adjust the default autovacuum
settings to better fit the workload and ensure
a more efficient execution of the process:
# disable autovacuum schedule based on scale factor
autovacuum_vacuum_scale_factor:0
autovacuum_analyze_scale_factor:0
# set thresholds based on approximate number of facts inserted
autovacuum_vacuum_threshold:<number of new facts each month>
autovacuum_analyze_threshold:<number of new facts each week>
When used as AWS RDS
AWS RDS Configuration
Most of the time, the default RDS configuration of PostgreSQL is sufficient. However, in some cases, it might be necessary to adjust some settings in the RDS Parameter Groups to improve performance. The following settings are recommended for FactCast instances running on production stages:
# hands over concurrency considerations to kernel
effective_io_concurrency:0
# tune accordingly, consider roughly 100mb running on a db.r5.2xlarge RDS instance
work_mem:100000
# the followings might vary, depending on your non-functional requirements
log_statement:'none'
log_min_duration_statement:500
default_statistics_target:100
# allows to deploy major version updates via blue/green deployments, significantly reducing downtime
rds.logical_replication:'1'