mapleFU commented on code in PR #197:
URL: https://github.com/apache/parquet-format/pull/197#discussion_r1318347248
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src/main/thrift/parquet.thrift:
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@@ -191,6 +191,73 @@ 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 field may be omitted if max_repetition_level is 0.
+ **/
+ 1: optional list<i64> repetition_level_histogram;
+ /**
+ * Same as repetition_level_histogram except for definition levels.
+ *
+ * This field may be omitted if max_definition_level is 0 or 1.
+ **/
+ 2: optional list<i64> definition_level_histogram;
+ }
+
+/**
+ * A structure for capturing metadata for estimating the unencoded,
+ * uncompressed size of data written. This is useful for readers to estimate
+ * how much memory is needed to reconstruct data in their memory model and for
+ * fine grained filter pushdown on nested structures (the histogram contained
+ * in this structure can help determine the number of nulls at a particular
+ * nesting level).
+ *
+ * Writers should populate all fields in this struct except for the exceptions
+ * listed per field.
+ */
+struct SizeStatistics {
+ /**
+ * The number of physical bytes stored for BYTE_ARRAY data values assuming
+ * no encoding. This is exclusive of the bytes needed to store the length of
+ * each byte array. In other words, this field is equivalent to the `(size
+ * of PLAIN-ENCODING the byte array values) - (4 bytes * number of values
+ * written)`. To determine unencoded sizes of other types readers can use
+ * schema information multiplied by the number of non-null and null values.
+ * The number of null/non-null values can be inferred from the histograms
+ * below.
+ *
+ * For example, if a column chunk is dictionary-encoded with dictionary
+ * ["a", "bc", "cde"], and a data page contains the indices [0, 0, 1, 2],
+ * then this value for that data page should be 7 (1 + 1 + 2 + 3).
+ *
+ * This field should only be set for types that use BYTE_ARRAY as their
+ * physical type.
+ */
+ 1: optional i64 unencoded_byte_array_data_bytes;
Review Comment:
> It also seems off to me that it assumes readers can't preserve dictionaries
Hmmm right maybe dictionary size should be take into account. It would be ok
for read dictionary into Plain data(like Binary / String), but read into
dictionary indices would require just dictionary raw-binary size. Should we
just reserve some info for dictionary in `ColumnChunk` @emkornfield (Or this is
not required)?
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