org.apache.drill.exec.physical.resultSet

Interface PullResultSetReader

All Known Implementing Classes:

PullResultSetReaderImpl

public interface PullResultSetReader

Iterates over the set of batches in a result set, providing a row set reader to iterate over the rows within each batch. Handles schema changes between batches. A typical use is to iterate over batches from an upstream operator. Protocol:

Protocol

1. Create an instance.
2. For each incoming batch:
   1. Call #start() to attach the batch. The associated BatchAccessor reports if the schema has changed.
   2. Call reader() to obtain a reader.
   3. Iterate over the batch using the reader.
   4. Call #release() to free the memory for the incoming batch. Or, to call #detach() to keep the batch memory.
3. Call close() after all batches are read.

• Create the result set reader via a specific subclass. If a query has a null result (no rows, no schema), the code which creates this class should instead indicate that no results are available. This class is only for the cases
• Call schema(), if desired, to obtain the schema for this result set.
• Call next() to advance to the first batch.
• If next() returns true, then call reader() to obtain a reader over rows. This reader also provides the batch schema.
• Use the reader to iterate over rows in the batch.
• Call next() to advance to the next batch and repeat.

The implementation may perform complex tasks behind the scenes: coordinate with the query runner (if remote), drive an operator (if within a DAG), etc. The implementation takes an interface that interfaces with the source of batches.

Designed to handle batches arriving from a single upstream operator. Uses Drill's strict form of schema identity: that not only must the column definitions match; the vectors must be identical from one batch to the next. If the vectors differ, then this class assumes a new schema has occurred, and will rebuild all the underlying readers, which can be costly.

Method Summary

Modifier and Type	Method and Description
void	close() Close this reader.
boolean	next() Advance to the next batch of data.
RowSetReader	reader() Obtain a reader to iterate over the rows of the batch.
TupleMetadata	schema() Return the schema for this result set.
int	schemaVersion()

Method Detail

next

boolean next()

Advance to the next batch of data. The iterator starts positioned before the first batch (but after obtaining a schema.)

Returns:

true if another batch is available, false if EOF

schema

TupleMetadata schema()

Return the schema for this result set.

schemaVersion

int schemaVersion()

reader

RowSetReader reader()

Obtain a reader to iterate over the rows of the batch. The return value will likely be the same reader each time, so that this call is optional after the first batch.

close

void close()

Close this reader. Releases any memory still assigned to any attached batch. Call #detach() first if you want to preserve the batch memory.