Enabling multi-threaded queries

Steps to enable multi-threading if excessive writes exist.

Multi-threaded queries are useful for a low-indexing volume with longer running queries.

Multi-threaded queries can offset the load of a query onto the CPU instead of writing and reading to disk. Benchmarking is recommended, multi-threaded queries do not always improve performance.

Use the CQL index management commands to set the number of queryExecutorThreads for the search index config:

Change the number of threads on an existing table:

ALTER SEARCH INDEX CONFIG ON healthcare.health_data SET config.queryExecutorThreads=4;

To view the pending search index config in XML format, use this CQL shell command:

DESCRIBE PENDING SEARCH INDEX CONFIG ON healthcare.health_data;

The results in XML:

<config>
  ...
<query>
    <maxBooleanClauses>1024</maxBooleanClauses>
    <filterCache class="solr.SolrFilterCache" highWaterMarkMB="2048" lowWaterMarkMB="1024"/>
    <enableLazyFieldLoading>true</enableLazyFieldLoading>
    <useColdSearcher>true</useColdSearcher>
    <maxWarmingSearchers>16</maxWarmingSearchers>
  ...
</query>
...
    <queryExecutorThreads>4</queryExecutorThreads>
</config>
  ...

Use the command to apply the pending changes to the search index:
```
RELOAD SEARCH INDEX ON healthcare.health_data;
```

To view the active search index in XML format:

DESCRIBE ACTIVE SEARCH INDEX CONFIG ON healthcare.health_data;