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Asked: 2026-05-10T15:59:07+00:00

We run full re-indexes every 7 days (i.e. creating the index from scratch) on

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We run full re-indexes every 7 days (i.e. creating the index from scratch) on our Lucene index and incremental indexes every 2 hours or so. Our index has around 700,000 documents and a full index takes around 17 hours (which isn’t a problem).

When we do incremental indexes, we only index content that has changed in the past two hours, so it takes much less time – around half an hour. However, we’ve noticed that a lot of this time (maybe 10 minutes) is spent running the IndexWriter.optimize() method.

The LuceneFAQ mentions that:

The IndexWriter class supports an optimize() method that compacts the index database and speeds up queries. You may want to use this method after performing a complete indexing of your document set or after incremental updates of the index. If your incremental update adds documents frequently, you want to perform the optimization only once in a while to avoid the extra overhead of the optimization.

…but this doesn’t seem to give any definition for what ‘frequently’ means. Optimizing is CPU intensive and VERY IO-intensive, so we’d rather not be doing it if we can get away with it. How much is the hit of running queries on an un-optimized index (I’m thinking especially in terms of query performance after a full re-index compared to after 20 incremental indexes where, say, 50,000 documents have changed)? Should we be optimising after every incremental index or is the performance hit not worth it?

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  1. Mat, since you seem to have a good idea how long your current process takes, I suggest that you remove the optimize() and measure the impact.

    Do many of the documents change in those 2 hour windows? If only a small fraction (50,000/700,000 is about 7%) are incrementally re-indexed, then I don’t think you are getting much value out of an optimize().

    Some ideas:

    • Don’t do an incremental optimize() at all. My experience says you are not seeing a huge query improvement anyway.
    • Do the optimize() daily instead of 2-hourly.
    • Do the optimize() during low-volume times (which is what the javadoc says).

    And make sure you take measurements. These kinds of changes can be a shot in the dark without them.

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