julianhyde commented on a change in pull request #1101: [CALCITE-2909] Optimize
Enumerable SemiJoin with lazy computation of innerLookup (WIP, blocked by
CALCITE-2937) (Ruben Quesada Lopez)
URL: https://github.com/apache/calcite/pull/1101#discussion_r267954959
##########
File path:
linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java
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@@ -1298,11 +1298,11 @@ private void closeInner() {
final EqualityComparer<TKey> comparer) {
return new AbstractEnumerable<TSource>() {
public Enumerator<TSource> enumerator() {
- final Enumerable<TKey> innerLookup =
- comparer == null
- ? inner.select(innerKeySelector).distinct()
- : inner.select(innerKeySelector).distinct(comparer);
-
+ // 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
+ final Enumerable<TKey> innerLookup = comparer == null
+ ? Linq4j.lazyEnumerable(() ->
inner.select(innerKeySelector).distinct())
+ : Linq4j.lazyEnumerable(() ->
inner.select(innerKeySelector).distinct(comparer));
Review comment:
Rather than inventing `LazyEnumerable`, could you change the type of
`innerLookup` from `Enumerable<TKey>` to `Supplier<Enumerable<TKey>>` or
(better) `Supplier<Enumerator<TKey>>` and use Guava's `MemoizingSupplier`
The goal, after all, is that the hash table is computed 0 times if not
needed, and 1 time if needed 1 or more times. A memoizing supplier achieves
that very well.
I don't see anything in the code that requires `innerLookup` to be an
Enumerable.
My concern with LazyEnumerable is that Enumerable is, by design, a lazy data
structure. When you invent another layer of laziness, it will be difficult to
figure out which of the 2 lazy layers to put new features. It is already
confusing enough.
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