gsmiller commented on code in PR #12560:
URL: https://github.com/apache/lucene/pull/12560#discussion_r1327828413
##########
lucene/expressions/src/java/org/apache/lucene/expressions/ExpressionFunctionValues.java:
##########
@@ -39,21 +39,21 @@ class ExpressionFunctionValues extends DoubleValues {
}
@Override
- public boolean advanceExact(int doc) {
+ public boolean advanceExact(int doc) throws IOException {
if (currentDoc == doc) {
return true;
}
+ for (DoubleValues v : functionValues) {
+ v.advanceExact(doc);
Review Comment:
There's a bit of indirection here, so maybe this will help:
1. `ExpressionFunctionValues` represents a computed expression value for a
given doc. This comes from any instance of `Expression` (e.g., from
`JavascriptCompiler#compile`). `Expression#getDoubleValuesSource` provides an
instance of `ExpressionValueSource`, which in turn provides
`ExpressionFunctionValues` when you call `#getValues` for a specific segment
context. `ExpressionFunctionValues#advanceExact` always returns true
(expressions produce results for all docs), and
`ExpressionFunctionValues#doubleValue` computes the value for the doc.
2. In `ExpressionValueSource#getValues`, we take all the expression's
arguments (which are all instances of `DoubleValues`) and wrap them with an
anonymous class provided by `#zeroWhenUnpositioned`. The main purpose of this
wrapper is to provide a value of `0` for any expression argument that isn't
available for a doc (i.e., `advanceExact` returned false for the doc). This PR
extends the functionality of this wrapper to lazily advance until the point
when `doubleValues` is called.
3. So without this PR, `ExpressionFunctionValues#advanceExact` is lazy but
once `ExpressionFunctionValues#doubleValues` is called, it advances all of the
arguments (regardless of whether-or-not that argument is ever needed for a
given doc). With this change, the `advanceExact` also becomes lazy, so it gets
deferred until a value is actually needed.
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