wgtmac commented on code in PR #1023:
URL: https://github.com/apache/parquet-mr/pull/1023#discussion_r1114230889
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parquet-hadoop/src/main/java/org/apache/parquet/filter2/statisticslevel/StatisticsFilter.java:
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@@ -289,8 +320,14 @@ public <T extends Comparable<T>> Boolean visit(Lt<T> lt) {
T value = lt.getValue();
- // drop if value <= min
- return stats.compareMinToValue(value) >= 0;
+ // we are looking for records where v < someValue
+ if (stats.compareMinToValue(value) >= 0) {
+ // drop if value <= min
+ return BLOCK_CANNOT_MATCH;
+ } else {
+ // if value > min, we must take it
+ return BLOCK_MUST_MATCH;
Review Comment:
> We have dictionary encoding but not for all the pages. We also have Bloom
filter.
Yes, that's true.
> Does it worth reading the dictionary to check if a value is in there
knowing if it doesn't we still want to check the Bloom filter?
In this case, the dictionary will not be read via `expandDictionary(meta)`
by `DictionaryFilter` if `hasNonDictionaryPages(meta)` returns true and will
not make performance worse. e.g.
https://github.com/apache/parquet-mr/blob/261f7d2679407c833545b56f4c85a4ae8b5c9ed4/parquet-hadoop/src/main/java/org/apache/parquet/filter2/dictionarylevel/DictionaryFilter.java#L388
> For the case of all the pages are dictionary encoded we should not have
Bloom filters therefore it doesn't really matter if we return BLOCK_MIGHT_MATCH
or BLOCK_MUST_MATCH in case we find the interested values in the dictionary.
It is difficult to make the trade-off here. If we only have one predicate,
then the dictionary will read any way, either by the DictionaryFilter or by
reading the data later if the row group cannot be dropped. However, if we have
other predicates that can drop the row group, then reading the dictionary here
by DictionaryFilter is worthless.
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