etseidl commented on code in PR #197:
URL: https://github.com/apache/parquet-format/pull/197#discussion_r1313547575
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src/main/thrift/parquet.thrift:
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@@ -191,6 +191,74 @@ enum FieldRepetitionType {
REPEATED = 2;
}
+/**
+ * A histogram of repetition and definition levels for either a page or column
+ * chunk.
+ *
+ * This is useful for:
+ * 1. Estimating the size of the data when materialized in
+ * memory
+ *
+ * 2. For filter push-down on nulls at various levels of nested
+ * structures and list lengths.
+ */
+struct RepetitionDefinitionLevelHistogram {
+ /**
+ * When present, there is expected to be one element corresponding to each
+ * repetition (i.e. size=max repetition_level+1) where each element
+ * represents the number of times the repetition level was observed in the
+ * data.
+ *
+ * This value should not be written if max_repetition_level is 0.
+ **/
+ 1: optional list<i64> repetition_level_histogram;
+ /**
+ * Same as repetition_level_histogram except for definition levels.
+ *
+ * This value should not be written if max_definition_level is 0 or 1.
Review Comment:
Now that I think about it some, I understand the first element of the
histogram for page `p` will be the same as `null_counts[p]`. The second element
of the histogram is `num_values[p] - def_hist[p][0]`, but `num_values` only
lives on the page header. It's not in the page indexes. so if you want the page
histogram without reading the page header I think you'd be out of luck. Am I
correct in my thinking? And does this argue for keeping the def histogram even
for the `max_level == 0` case? I think it will not be possible to get page size
estimates using only the indexes otherwise.
Oh, but in the case where `max_rep == 0`, `num_rows == num_values`, and the
former can be recovered from the offset index. If `max_rep > 0`, then I think
`max_def` has to be at least `1`. So never mind.
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