Collecting slow queries

The node_slow_log retains query information of long-running CQL statements to help you identify slow queries on a cluster to easily find and tune poorly performing queries.

The node_slow_log table collects information about slow queries on a node and retains query information of long-running CQL statements to help you identify slow queries on a cluster to find and tune poorly performing queries.

The location of the dse.yaml file depends on the type of installation:

Installer-Services	/etc/dse/dse.yaml
Package installations	/etc/dse/dse.yaml
Installer-No Services	`install_location`/resources/dse/conf/dse.yaml
Tarball installations	`install_location`/resources/dse/conf/dse.yaml

Pending inserts into the node_slow_log might still be processed after the service has been disabled. You can enable and disable the service. After the service is disabled, the logging of queries that take longer than the specified threshold is stopped. However, disabling the logging does not flush the pending write queue, a background thread eventually processes everything.

Procedure

You can collect statements that are issued when the query exceeds a specified time threshold.
- To permanently enable collecting information on slow queries, edit the dse.yaml file and uncomment the cql_slow_log_threshold_ms parameter, and define values for the CQL slow log settings:
```
# CQL slow log settings
enabled: true
threshold_ms: 100
ttl_seconds: 86400
async_writers: 1
```
- To temporarily enable collecting information on slow queries that exceeded the threshold of 200 milliseconds:
```
$ dsetool perf cqlslowlog enable
$ dsetool perf cqlslowlog 200
```
  After you collect information using this temporarily set threshold, you can run a script to view queries that took longer with this threshold than the previously set threshold. For example:
```
$ cqlsh 
cqlsh> use dse_perf; 
cqlsh:dse_perf> select * from node_slow_log;
...
```

You can export slow queries using the CQL copy command:

cqlsh:dse_perf> COPY node_slow_log ( date, commands, duration ) TO 'slow_queries.csv' WITH HEADER = true;