Perform case-insensitive sorting in Solr

To perform case-insensitive sorting in Solr, you can create a custom field type that applies a lowercasing filter, so that the text is indexed in lowercase. This way, when you sort on this field, it will effectively ignore the case of the original text.

Here's an example of how you can create a custom field type that ignores case sensitivity:

<fieldType name="text_sort_ignore_case" class="solr.TextField" sortMissingLast="true" omitNorms="true">
  <analyzer>
    <tokenizer class="solr.KeywordTokenizerFactory"/>
    <filter class="solr.LowerCaseFilterFactory"/>
  </analyzer>
</fieldType>

Now, you can use this custom field type for fields that need case-insensitive sorting:

<field name="title_sort_ignore_case" type="text_sort_ignore_case" indexed="true" stored="false" multiValued="false"/>

When you want to sort your search results, you can use the sort parameter in your query:

• For ascending order: sort=title_sort_ignore_case asc
• For descending order: sort=title_sort_ignore_case desc

This will sort your search results in ascending or descending order while ignoring case sensitivity.

Applying a LowerCaseFilter in Solr does have a small impact on performance, but it is generally negligible. The LowerCaseFilter is a simple and lightweight filter that converts text to lowercase during the indexing process. The performance impact occurs only once during indexing and not during every search query.

However, if you have a very large dataset and you are concerned about the performance, you can always evaluate the filter's impact by benchmarking your Solr instance with and without the LowerCaseFilter applied. This will give you a better understanding of how the filter affects the performance of your specific use case.

In most situations, the benefit of case-insensitive sorting outweighs the minor performance hit introduced by the LowerCaseFilter. The filter is widely used and considered a standard part of text analysis pipelines in Solr and other search platforms.