org.apache.drill.exec.physical.impl.scan

Interface ScanOperatorEvents

All Known Implementing Classes:

ColumnsScanFramework, FileScanFramework, ManagedScanFramework, ScanEventListener

public interface ScanOperatorEvents

Interface to the set of readers, and reader schema, that the scan operator manages. The reader factory creates and returns the readers to use for the scan, as determined by the specific physical plan. The reader factory also translates from the select list provided in the physical plan to the actual columns returned from the scan operator. The translation is reader-specific; this interface allows the scan operator to trigger various lifecycle events.

This interface decouples the scan implementation from the generic tasks needed to implement Drill's Volcano iterator protocol for operators, and Drill's schema and batch semantics. A scan implementation need only implement this interface to add plugin-specific scan behavior.

While this interface allows a wide variety of implementations, the intent is that most actual scanners will use the "managed" framework that handles the routine projection, vector management and other tasks that tend to be common across scanners. See ScanSchemaOrchestrator for the managed framework.

Method Summary

Modifier and Type	Method and Description
void	bind(OperatorContext context) Build the scan-level schema from the physical operator select list.
void	close() Called when the scan operator itself is closed.
RowBatchReader	nextReader() A scanner typically readers multiple data sources (such as files or file blocks.) A batch reader handles each read.

Method Detail

bind

void bind(OperatorContext context)

Build the scan-level schema from the physical operator select list. The operator context is provided to allow access to the user name, to options, and to other information that might influence schema resolution.

After this call, the schema manager should be ready to build a reader-specific schema for each reader as it is opened.

Parameters:

context - the operator context for the scan operator

nextReader

RowBatchReader nextReader()

A scanner typically readers multiple data sources (such as files or file blocks.) A batch reader handles each read. This method returns the next reader in whatever sequence that this scan defines.

The preferred implementation is to create each batch reader in this call to minimize resource usage. Production queries may read thousands of files or blocks, so incremental reader creation can be far more efficient than creating readers at the start of the scan.

Returns:

a batch reader for one of the scan elements within the scan physical plan for this scan operator

close

void close()

Called when the scan operator itself is closed. Indicates that no more readers are available.