Monitor performance with the Query Store

Applies to: Azure Database for PostgreSQL 9.6 and 10

The Query Store feature in Azure Database for PostgreSQL provides a way to track query performance over time. Query Store simplifies performance troubleshooting by helping you quickly find the longest running and most resource-intensive queries. Query Store automatically captures a history of queries and runtime statistics, and it retains them for your review. It separates data by time windows so that you can see database usage patterns. Data for all users, databases, and queries is stored in a database named azure_sys in the Azure Database for PostgreSQL instance.

Important

Do not modify the azure_sys database or its schemas. Doing so will prevent Query Store and related performance features from functioning correctly.

Enabling Query Store

Query Store is an opt-in feature, so it isn't active by default on a server. The store is enabled or disabled globally for all the databases on a given server and cannot be turned on or off per database.

Enable Query Store using the Azure portal

  1. Sign in to the Azure portal and select your Azure Database for PostgreSQL server.
  2. Select Server Parameters in the Settings section of the menu.
  3. Search for the pg_qs.query_capture_mode parameter.
  4. Set the value to TOP and Save.

To enable wait statistics in your Query Store:

  1. Search for the pgms_wait_sampling.query_capture_mode parameter.
  2. Set the value to ALL and Save.

Alternatively you can set these parameters using the Azure CLI.

az postgres server configuration set --name pg_qs.query_capture_mode --resource-group myresourcegroup --server mydemoserver --value TOP
az postgres server configuration set --name pgms_wait_sampling.query_capture_mode --resource-group myresourcegroup --server mydemoserver --value ALL

Allow up to 20 minutes for the first batch of data to persist in the azure_sys database.

Information in Query Store

Query Store has two stores:

  • A runtime stats store for persisting the query execution statistics information.
  • A wait stats store for persisting wait statistics information.

Common scenarios for using Query Store include:

  • Determining the number of times a query was executed in a given time window
  • Comparing the average execution time of a query across time windows to see large deltas
  • Identifying longest running queries in the past X hours
  • Identifying top N queries that are waiting on resources
  • Understanding wait nature for a particular query

To minimize space usage, the runtime execution statistics in the runtime stats store are aggregated over a fixed, configurable time window. The information in these stores is visible by querying the query store views.

The following query returns information about queries in Query Store:

SELECT * FROM query_store.qs_view; 

Or this query for wait stats:

SELECT * FROM query_store.pgms_wait_sampling_view;

Finding wait queries

Wait event types combine different wait events into buckets by similarity. Query Store provides the wait event type, specific wait event name, and the query in question. Being able to correlate this wait information with the query runtime statistics means you can gain a deeper understanding of what contributes to query performance characteristics.

Here are some examples of how you can gain more insights into your workload using the wait statistics in Query Store:

Observation Action
High Lock waits Check the query texts for the affected queries and identify the target entities. Look in Query Store for other queries modifying the same entity, which is executed frequently and/or have high duration. After identifying these queries, consider changing the application logic to improve concurrency, or use a less restrictive isolation level.
High Buffer IO waits Find the queries with a high number of physical reads in Query Store. If they match the queries with high IO waits, consider introducing an index on the underlying entity, in order to do seeks instead of scans. This would minimize the IO overhead of the queries. Check the Performance Recommendations for your server in the portal to see if there are index recommendations for this server that would optimize the queries.
High Memory waits Find the top memory consuming queries in Query Store. These queries are probably delaying further progress of the affected queries. Check the Performance Recommendations for your server in the portal to see if there are index recommendations that would optimize these queries.

Configuration options

When Query Store is enabled it saves data in 15-minute aggregation windows, up to 500 distinct queries per window.

The following options are available for configuring Query Store parameters.

Parameter Description Default Range
pg_qs.query_capture_mode Sets which statements are tracked. none none, top, all
pg_qs.max_query_text_length Sets the maximum query length that can be saved. Longer queries will be truncated. 6000 100 - 10K
pg_qs.retention_period_in_days Sets the retention period. 7 1 - 30
pg_qs.track_utility Sets whether utility commands are tracked on on, off

The following options apply specifically to wait statistics.

Parameter Description Default Range
pgms_wait_sampling.query_capture_mode Sets which statements are tracked for wait stats. none none, all
Pgms_wait_sampling.history_period Set the frequency, in milliseconds, at which wait events are sampled. 100 1-600000

Note

pg_qs.query_capture_mode supersedes pgms_wait_sampling.query_capture_mode. If pg_qs.query_capture_mode is NONE, the pgms_wait_sampling.query_capture_mode setting has no effect.

Use the Azure portal or Azure CLI to get or set a different value for a parameter.

Views and functions

View and manage Query Store using the following views and functions. Anyone in the PostgreSQL public role can use these views to see the data in Query Store. These views are only available in the azure_sys database.

Queries are normalized by looking at their structure after removing literals and constants. If two queries are identical except for literal values, they will have the same hash.

query_store.qs_view

This view returns all the data in Query Store. There is one row for each distinct database ID, user ID, and query ID.

Name Type References Description
runtime_stats_entry_id bigint ID from the runtime_stats_entries table
user_id oid pg_authid.oid OID of user who executed the statement
db_id oid pg_database.oid OID of database in which the statement was executed
query_id bigint   Internal hash code, computed from the statement's parse tree
query_sql_text Varchar(10000)   Text of a representative statement. Different queries with the same structure are clustered together; this text is the text for the first of the queries in the cluster.
plan_id bigint ID of the plan corresponding to this query, not available yet
start_time timestamp Queries are aggregated by time buckets - the time span of a bucket is 15 minutes by default. This is the start time corresponding to the time bucket for this entry.
end_time timestamp End time corresponding to the time bucket for this entry.
calls bigint   Number of times the query executed
total_time double precision   Total query execution time, in milliseconds
min_time double precision Minimum query execution time, in milliseconds
max_time double precision Maximum query execution time, in milliseconds
mean_time double precision Mean query execution time, in milliseconds
stddev_time double precision Standard deviation of the query execution time, in milliseconds
rows bigint   Total number of rows retrieved or affected by the statement
shared_blks_hit bigint   Total number of shared block cache hits by the statement
shared_blks_read bigint Total number of shared blocks read by the statement
shared_blks_dirtied bigint   Total number of shared blocks dirtied by the statement
shared_blks_written bigint   Total number of shared blocks written by the statement
local_blks_hit bigint Total number of local block cache hits by the statement
local_blks_read bigint   Total number of local blocks read by the statement
local_blks_dirtied bigint   Total number of local blocks dirtied by the statement
local_blks_written bigint   Total number of local blocks written by the statement
temp_blks_read bigint   Total number of temp blocks read by the statement
temp_blks_written bigint   Total number of temp blocks written by the statement
blk_read_time double precision   Total time the statement spent reading blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)
blk_write_time double precision   Total time the statement spent writing blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)

query_store.query_texts_view

This view returns query text data in Query Store. There is one row for each distinct query_text.

Name Type Description
query_text_id bigint ID for the query_texts table
query_sql_text Varchar(10000)   Text of a representative statement. Different queries with the same structure are clustered together; this text is the text for the first of the queries in the cluster.

query_store.pgms_wait_sampling_view

This view returns wait events data in Query Store. There is one row for each distinct database ID, user ID, query ID, and event.

Name Type References Description
user_id oid pg_authid.oid OID of user who executed the statement
db_id oid pg_database.oid OID of database in which the statement was executed
query_id bigint   Internal hash code, computed from the statement's parse tree
event_type text   The type of event for which the backend is waiting
event text The wait event name if backend is currently waiting
calls Integer Number of the same event captured

Functions

Query_store.qs_reset() returns void

qs_reset discards all statistics gathered so far by Query Store. This function can only be executed by the server admin role.

Query_store.staging_data_reset() returns void

staging_data_reset discards all statistics gathered in memory by Query Store (that is, the data in memory that has not been flushed yet to the database). This function can only be executed by the server admin role.

Limitations and known issues

  • If a PostgreSQL server has the parameter default_transaction_read_only on, Query Store cannot capture data.
  • Query Store functionality can be interrupted if it encounters long Unicode queries (>= 6000 bytes).

Next steps