Slow Solr Queries Debugging

Understanding and Fixing Slow Drupal Solr Queries

Your Drupal site is slow when searching. You check the Solr debug output and discover that the query itself runs in 200-300ms — but faceting takes 1,500-2,000ms or more. The bottleneck is not Solr's search — it is all the extra work Drupal asks Solr to do.

Why Drupal Queries Are Slow

1. The *:* Query Pattern

Drupal often sends {!boost b=boost_document} *:* — meaning "match every single document." Even with filter queries narrowing results, Solr still has to evaluate the entire index first.

2. Too Many Facets

Drupal's Facet API requests facet counts for every active facet field. Each facet must count every term across every matching document. With 10+ facets and facet.limit=-1 (return all values), this is the main performance killer.

3. FCS Method (Slow)

Solr's default FieldCache Scanning (FCS) method scans the entire field value space. Modern alternatives like JSON facets or streaming facets are much faster.

4. Excessive Filters

Dozens of fq parameters — especially negations like (*:* -field:value) and range queries — each add cost.

Fixes in Drupal

A. Reduce Active Facets

Go to Configuration > Search API > View Modes and disable facets that are not visible on the page. Every active facet triggers Solr work, even if Drupal does not render it.

B. Limit Facet Results

In Facet API settings, change facet.limit from -1 (return everything) to a reasonable number:

facet.limit = 50
facet.mincount = 1

C. Use the JSON Facet API

If your Solr version is 7.x or newer, switch Drupal's Search API backend to use the JSON Facet API. It is faster, parallelized, and supports streaming facet counts.

D. Enable Caching

Enable Drupal's Search API query cache and facet cache to avoid recomputing identical facets for repeated queries.

Fixes in Solr

A. Enable DocValues on Facet Fields

Ensure all fields used for faceting have docValues="true" in your schema:

<field name="field_category" type="string" stored="true"
       indexed="true" docValues="true"/>

DocValues make faceting dramatically faster.

B. Use facet.threads

Parallelize facet computation:

facet.threads=4

C. Enable Filter Cache

In solrconfig.xml, ensure you have a properly sized filter cache:

<filterCache class="solr.FastLRUCache" size="512"
             initialSize="512" autowarmCount="128"/>

D. Switch to JSON Facets

Replace classic facet.field parameters with JSON facets:

json.facet={
  categories: { type: "terms", field: "field_category", limit: 50 },
  formats: { type: "terms", field: "field_format", limit: 50 }
}

Optimization Checklist

Before vs After

Before (slow):

q={!boost b=boost_document} *:*&facet.limit=-1&facet.field=field_a&facet.field=field_b...

After (fast):

q=*:*&fq=available:true&facet.limit=50&facet.threads=4&
json.facet={
  categories:{type:terms,field:field_category,limit:50},
  formats:{type:terms,field:field_format,limit:50}
}

Need help optimizing your Drupal search performance? Reach out to us at support@opensolr.com — we can analyze your query patterns and recommend the right optimizations.