ChrisHegarty commented on code in PR #16418:
URL: https://github.com/apache/lucene/pull/16418#discussion_r3664786339
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lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java:
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@@ -324,6 +338,43 @@ public boolean getReaderPooling() {
return readerPooling;
}
+ /**
+ * When enabled, a doc-values update that only sets values (no removals) is
written as a sparse
+ * "delta" generation holding just the updated documents and overlaid on the
existing column at
+ * read time, rather than rewriting the whole column. This trades some read
cost for much lower
+ * write amplification on frequently updated fields. Enabled by default;
pass {@code false} to
+ * keep the classic full-column rewrite.
+ *
+ * <p>A commit whose segments carry sparse delta generations is written at a
bumped segments-file
+ * version, so a Lucene version that predates this feature refuses to open
it rather than
+ * misreading a delta as the whole column. A segment merge (including a
force-merge) flattens the
+ * overlay back to a plain column, so once all overlays are merged away
older versions can read
+ * the index again. Only takes effect when IndexWriter is first created.
+ *
+ * @lucene.experimental
+ */
+ public IndexWriterConfig setIncrementalDocValuesUpdates(boolean
incrementalDocValuesUpdates) {
Review Comment:
Minor API thought: do we need both `setIncrementalDocValuesUpdates(boolean)`
and `setMaxDocValuesDeltaGenerations(int)`? It seems like a single setting
would cover both `setMaxDocValuesDeltaGenerations(0)` disables the feature,
anything `> 0` enables it with that depth. Avoids the question of what happens
when the two disagree, and one fewer knob for users to reason about.
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