wesm commented on a change in pull request #7534:
URL: https://github.com/apache/arrow/pull/7534#discussion_r453366215
##########
File path: rust/datafusion/README.md
##########
@@ -69,5 +69,15 @@ DataFusion includes a simple command-line interactive SQL
utility. See the [CLI
- [x] Parquet primitive types
- [ ] Parquet nested types
-# Examples
+# Supported SQL
+This library currently supports the following SQL constructs:
+
+* `CREATE EXTERNAL TABLE X STORED AS PARQUET LOCATION '...';` to register a
table's locations
+* `SELECT ... FROM ...` together with any expression
+* `ALIAS` to name an expression
+* `CAST` to change types, including e.g. `Timestamp(Nanosecond, None)`
+* most mathematical unary and binary expressions such as `+`, `/`, `sqrt`,
`tan`, `>=`.
+* `WHERE` to filter
+* `GROUP BY` together with one of the following aggregations: `MIN`, `MAX`,
`COUNT`, `SUM`, `AVG`
+* `ORDER BY` together with an expression and optional `DESC`
Review comment:
This shouldn't be here
##########
File path: cpp/src/parquet/arrow/reader.h
##########
@@ -151,8 +151,10 @@ class PARQUET_EXPORT FileReader {
int i, std::shared_ptr<::arrow::ChunkedArray>* out) = 0;
/// \brief Return a RecordBatchReader of row groups selected from
row_group_indices, the
- /// ordering in row_group_indices matters.
- /// \returns error Status if row_group_indices contains invalid index
+ /// ordering in row_group_indices matters. Each row group will held in
memory until
+ /// each slice has been yielded as a RecordBatch, at which point the
next row group
+ /// will be loaded. FileReaders must outlive their RecordBatchReaders.
Review comment:
Probably want to add a TODO here that we want to fix this (this is a
problem for memory use)
##########
File path: cpp/src/parquet/arrow/reader.cc
##########
@@ -794,8 +773,35 @@ Status FileReaderImpl::GetRecordBatchReader(const
std::vector<int>& row_group_in
END_PARQUET_CATCH_EXCEPTIONS
}
- return RowGroupRecordBatchReader::Make(row_group_indices, column_indices,
this,
- reader_properties_.batch_size(), out);
+ using ::arrow::RecordBatchIterator;
+
+ // NB: This lambda will be invoked lazily whenever a new row group must be
+ // scanned, so it must capture `column_indices` by value (it will not
+ // otherwise outlive the scope of `GetRecordBatchReader()`). `this` is a
non-owning
+ // pointer so we are relying on the parent FileReader outliving this
RecordBatchReader.
+ auto row_group_index_to_batch_iterator =
+ [column_indices, this](const int* i) ->
::arrow::Result<RecordBatchIterator> {
+ std::shared_ptr<::arrow::Table> table;
+ RETURN_NOT_OK(RowGroup(*i)->ReadTable(column_indices, &table));
Review comment:
It isn't great to be reading the whole row group into memory before
returning it in chunks, is there an issue about materializing only a chunk at a
time (or a background actor-type reader that will prepare the materialize the
next chunk asynchronously in between calls to `Next`)?
##########
File path: cpp/src/parquet/arrow/reader.h
##########
@@ -163,8 +165,11 @@ class PARQUET_EXPORT FileReader {
/// \brief Return a RecordBatchReader of row groups selected from
/// row_group_indices, whose columns are selected by column_indices. The
/// ordering in row_group_indices and column_indices matter.
+ /// Each row group will held in memory until each slice has been yielded
as a
+ /// RecordBatch, at which point the next row group will be loaded.
FileReaders must
+ /// outlive their RecordBatchReaders.
Review comment:
Add a TODO that this is considered to be a problem
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