asereda-gs commented on a change in pull request #1101: [CALCITE-2909] Optimize
Enumerable SemiJoin with lazy computation of innerLookup (Ruben Quesada Lopez)
URL: https://github.com/apache/calcite/pull/1101#discussion_r264328064
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File path:
linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java
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@@ -1297,15 +1297,21 @@ private void closeInner() {
final Function1<TInner, TKey> innerKeySelector,
final EqualityComparer<TKey> comparer) {
return new AbstractEnumerable<TSource>() {
+ private Enumerable<TKey> innerLookup = null;
+ // CALCITE-2909 Delay the computation of the innerLookup until the
moment when we are sure
+ // that it will be really needed, i.e. when the first outer enumerator
item is processed
+ private Enumerable<TKey> getInnerLookup() {
Review comment:
I wonder if it makes more sense to create a generic `LazyEnumerable`. So the
code becomes :
```java
return Linq4j.lazy(() -> inner.select(innerKeySelector).distinct());
```
This is similar idea to Guava
[Supplier.memoize](https://google.github.io/guava/releases/23.0/api/docs/com/google/common/base/Suppliers.html#memoize-com.google.common.base.Supplier-)
or
[Observable.defer()](http://reactivex.io/RxJava/2.x/javadoc/io/reactivex/Observable.html#defer-java.util.concurrent.Callable-).
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