DocValues

DocValues, short for Document Values, is a feature in Apache Lucene, a high-performance, full-featured text search engine library. It is designed to provide efficient access to field values in a Lucene index. DocValues are particularly useful for fields that are frequently accessed in queries, such as those used for sorting, aggregations, and filtering.

DocValues are stored in a columnar format, which allows for efficient retrieval of individual field values. This is in contrast to the traditional row-based storage used for document fields in Lucene. The columnar format enables faster access to specific field values, as it minimizes the amount of data that needs to be read from disk.

There are several types of DocValues, including numeric, binary, and sorted. Numeric DocValues are used for fields that contain numeric data, such as integers or floating-point numbers. Binary DocValues are used for fields that contain binary data, such as strings or byte arrays. Sorted DocValues are used for fields that contain sorted data, such as dates or timestamps.

DocValues are indexed at the time of document indexing, and the resulting data is stored in a separate file in the Lucene index. This allows for efficient access to field values without the need to load the entire document into memory.

In summary, DocValues are a powerful feature in Apache Lucene that provides efficient access to field values in a Lucene index. They are particularly useful for fields that are frequently accessed in queries, and they are stored in a columnar format to enable faster retrieval of individual field values.