KazydubB commented on a change in pull request #1954: DRILL-7509: Incorrect
TupleSchema is created for DICT column when querying Parquet files
URL: https://github.com/apache/drill/pull/1954#discussion_r366847315
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File path:
exec/vector/src/main/java/org/apache/drill/exec/record/metadata/MetadataUtils.java
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@@ -187,6 +187,10 @@ public static ColumnMetadata newMapArray(String name,
TupleMetadata schema) {
return new MapColumnMetadata(name, DataMode.REPEATED, (TupleSchema)
schema);
}
+ public static DictColumnMetadata newDictArray(String name, TupleMetadata
schema) {
+ return new DictColumnMetadata(name, DataMode.REPEATED, (TupleSchema)
schema);
Review comment:
Let me explain: Dict stores `key` and `value` `ColumnMetadata`s in
`TupleSchema` - the way Map's members are stored - but there's a validation for
each of the fields (name, type). Dict does not contain a Map, it stores its
`key` and `value` in its `TupleSchema schema` field.
When `TupleMetadata` is constructed for Parquet table as part of table
metadata, we loop over each Parquet field's `SchemaPath` representation (leaf
fields, e.g. `` `mapcol`.`map`.`key` ``, `` `structcol`.`b` `` with the last
field in schema being a primitive). Such named segments are treated as either a
(Drill's) `MAP` or `DICT`, depending on parent segment type.
(Parquet's) `MAP` is represented as a group (note, that nested group's name,
`key_value` below, can be different, based on the system which produced the
Parquet file):
```
<map-repetition> group <name> (MAP) {
repeated group key_value {
required <key-type> key;
<value-repetition> <value-type> value;
}
}
```
and before changes in the PR, when `TupleMetadata` was being created for the
table, if `DICT` column was encountered, it included a nested `key_value` group
as Drill's `MAP` which then contained `key` and `value` fields. Thus, there is
a need to skip this segment if we know that its parent's type is `DICT` to have
correct `ColumnMetadata` for the `DICT` field.
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