Manage boolean clauses with maxBooleanClauses

Issue

• How to address the "Query contains too many nested clauses" OR "Too many clauses in boolean query" problems in Solr or Fusion?
• How to configure the maxBooleanClauses parameter in Solr or Fusion?

Diagnosis

This article focuses on the challenge of a boolean clause reaching its configured limit, particularly when encountering any of the following error messages in relation to a query.

messages": [
"Too many clauses in boolean query: encountered=3 configured in solrconfig.xml via maxBooleanClauses=2"
],

This error message is received when a boolean clause exceeds the maxBooleanClauses limit configured in solrconfig.xml for a specific collection.

message": "Query contains too many nested clauses; maxClauseCount is set to 1024",
"messages": [
"Query contains too many nested clauses; maxClauseCount is set to 1024"
],

This error message is received when a boolean clause exceeds the global maxBooleanClauses global limit configured in solr.xml.

Environment:

Fusion 5.8.0 and above

Solr 9.0 and above

Note: The proposed solution is applicable to other Fusion and Solr versions as well where the maxBooleanClauses is configured, but clients have been experiencing the issue more frequently with Fusion 5.8.0 due to changes in Solr 9.

Cause

The error message indicates that the query you're executing is generating more clauses than the configured maxBooleanClauses limit.

The primary reason for hitting the maxBooleanClause limit in Solr is often the usage of complex query structures, including multiple terms and boolean operators, especially in dynamically generated queries. Additionally, indexing large datasets with fields containing numerous unique values and applying extensive faceting or filtering operations can contribute to queries exceeding the configured maxBooleanClause threshold.

Solr's default behavior when dealing with 'maxBooleanClauses' has changed in Solr 9 to reduce the risk of exponential query expansion when dealing with pathological query strings. A default upper limit of 1024 clauses (the same default prior to Solr 7.0) is now enforced at the node level, and can be overridden in solr.xml.

For reference : https://solr.apache.org/guide/solr/latest/upgrade-notes/major-changes-in-solr-9.html

solr.xml maxBooleanClauses is now enforced recursively. Users who upgrade from prior versions of Solr may find that some requests involving complex internal query structures (Example: long query strings using edismax with many qf and pf fields that include query time synonym expansion) which worked in the past now hit this limit and fail. Users in this situation are advised to consider the complexity of their queries/configuration, and increase the value of maxBooleanClauses if warranted.

Resolution:

To resolve this issue, you can consider the following options:

1. Increase the maxBooleanClauses limit
2. Optimize your query to ensure it generates a number of clauses within the set limit

1. Increase the maxBooleanClauses limit

a) If the collection level maxBooleanClauses limit exceeds, you can increase the value in the solrconfig.xml.

<maxBooleanClauses>${solr.max.booleanClauses:2}</maxBooleanClauses>

Note: This per-collection limit is restricted by the upper-bound of the global limit in solr.xml. See SOLR-13336 for more details.

b) If the global maxBooleanClauses limit exceeds, you can increase the value in solr.xml or set the        -Dsolr.max.booleanClauses value in solr.

• In Fusion 5, the solr.xml file is located at /opt/solr-9.1.1/server/solr/solr.xml within the Solr pod container. The default value is 1024. You can change this value based on your requirements.
  
  

  <int name="maxBooleanClauses">${solr.max.booleanClauses:1024}</int>

• You can set the -Dsolr.max.booleanClauses=#### value to the SOLR_OPTS property of the fusion-solr StatefulSet (refer to the below screenshots). This setting will override all solrconfigs.
  

   
• 

After successfully updating the value, you can verify the property in the Solr UI.

Note: Increasing the maxBooleanClause value in Solr can also lead to increased memory usage and potential performance degradation due to larger query structures, impacting overall system efficiency. It's essential to balance this configuration based on specific use cases and hardware capabilities.

2. Optimize your query to ensure it generates a number of clauses within the set limit

In scenarios where long queries are common and you're concerned about hitting the maxBooleanClauses limit in Solr, there are several proactive measures you can take to mitigate the risk:

• Utilise filter queries (fq) for non-scoring conditions to offload them from the main query.
• Ensure your queries are as specific as possible, minimizing unnecessary terms or conditions.
• Choose and configure Boolean operators (AND, OR) wisely to avoid unnecessary clause multiplication.
• Implement result pagination to limit the number of results returned per query by using rows and start parameters to fetch results in chunks rather than fetching all at once.