Document dictionary page position (#177)

README.md

@@ -191,6 +191,13 @@ header and readers can skip over pages they are not interested in.  The data for
 page follows the header and can be compressed and/or encoded.  The compression and
 encoding is specified in the page metadata.
 
+A column chunk might be partly or completely dictionary encoded. It means that
+dictionary indexes are saved in the data pages instead of the actual values. The
+actual values are stored in the dictionary page. See details in
+[Encodings.md](https://github.com/apache/parquet-format/blob/master/Encodings.md#dictionary-encoding-plain_dictionary--2-and-rle_dictionary--8).
+The dictionary page must be placed at the first position of the column chunk. At
+most one dictionary page can be placed in a column chunk.
+
 Additionally, files can contain an optional column index to allow readers to
 skip pages more efficiently. See [PageIndex.md](PageIndex.md) for details and
 the reasoning behind adding these to the format.

src/main/thrift/parquet.thrift

@@ -534,6 +534,11 @@ struct IndexPageHeader {
   // TODO
 }
 
+/**
+ * The dictionary page must be placed at the first position of the column chunk
+ * if it is partly or completely dictionary encoded. At most one dictionary page
+ * can be placed in a column chunk.
+ **/
 struct DictionaryPageHeader {
   /** Number of values in the dictionary **/
   1: required i32 num_values;