Michael McCandless created LUCENE-8574:
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Summary: ExpressionFunctionValues should cache per-hit value
Key: LUCENE-8574
URL: https://issues.apache.org/jira/browse/LUCENE-8574
Project: Lucene - Core
Issue Type: Bug
Affects Versions: 7.5, master (8.0)
Reporter: Michael McCandless
The original version of {{ExpressionFunctionValues}} had a simple per-hit
cache, so that nested expressions that reference the same common variable would
compute the value for that variable the first time it was referenced and then
use that cached value for all subsequent invocations, within one hit. I think
it was accidentally removed in LUCENE-7609?
This is quite important if you have non-trivial expressions that reference the
same variable multiple times.
E.g. if I have these expressions:
{noformat}
x = c + d
c = b + 2
d = b * 2{noformat}
Then evaluating x should only cause b's value to be computed once (for a given
hit), but today it's computed twice. The problem is combinatoric if b then
references another variable multiple times, etc.
I think to fix this we just need to restore the per-hit cache?
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