==================================== Supporting DELETE and UPDATE

The Presto engine provides APIs to support row-level SQL DELETE and UPDATE. To implement DELETE or UPDATE, a connector must:

• Layer an UpdatablePageSource on top of the connector's ConnectorPageSource
• Define ConnectorMetadata methods to get a rowId column handle
• Start the operation using beginUpdate() or beginDelete()
• Finish the operation using finishUpdate() or finishDelete()

DELETE and UPDATE Data Flow

DELETE and UPDATE have a similar flow:

• For each split, the connector will create an UpdatablePageSource instance, layered over the connector's ConnectorPageSource, to read pages on behalf of the Presto engine, and to write deletions or updates to the underlying data store.
• The connector's UpdatablePageSource.getNextPage() implementation fetches the next page from the underlying ConnectorPageSource, optionally reformats the page, and returns it to the Presto engine.
• The Presto engine performs filtering and projection on the page read, producing a page of filtered, projected results.
• The Presto engine passes that filtered, projected page of results to the connector's UpdatablePageSource deleteRows() or updateRows() method. Those methods persist the deletions or updates in the underlying data store.
• When all the pages for a specific split have been processed, the Presto engine calls UpdatablePageSource.finish(), which returns a Collection<Slice> of fragments representing connector-specific information about the rows processed by the calls to deleteRows or updateRows.
• When all pages for all splits have been processed, the Presto engine calls ConnectorMetadata.finishDelete() or finishUpdate, passing a collection containing all the fragments from all the splits. The connector does what is required to finalize the operation, for example, committing the transaction.

The rowId Column Handle Abstraction

The Presto engine and connectors use a rowId column handle abstraction to agree on the identities of rows to be updated or deleted. The rowId column handle is opaque to the Presto engine. Depending on the connector, the rowId column handle abstraction could represent several physical columns.

The rowId Column Handle for DELETE

The Presto engine identifies the rows to be deleted using a connector-specific rowId column handle, returned by the connector's ConnectorMetadata.getDeleteRowIdColumnHandle() method, whose full signature is::

ColumnHandle getDeleteRowIdColumnHandle(
    ConnectorSession session,
    ConnectorTableHandle tableHandle)

The rowId Column Handle for UPDATE

The Presto engine identifies rows to be updated using a connector-specific rowId column handle, returned by the connector's ConnectorMetadata.getUpdateRowIdColumnHandle() method. In addition to the columns that identify the row, for UPDATE the rowId column will contain any columns that the connector requires in order to perform the UPDATE operation.

UpdatablePageSource API

As mentioned above, to support DELETE or UPDATE, the connector must define a subclass of UpdatablePageSource, layered over the connector's ConnectorPageSource. The interesting methods are:

• Page getNextPage(). When the Presto engine calls getNextPage(), the UpdatablePageSource calls its underlying ConnectorPageSource.getNextPage() method to get a page. Some connectors will reformat the page before returning it to the Presto engine.

• void deleteRows(Block rowIds). The Presto engine calls the deleteRows() method of the same UpdatablePageSource instance that supplied the original page, passing a block of rowIds, created by the Presto engine based on the column handle returned by ConnectorMetadata.getDeleteRowIdColumnHandle()

• void updateRows(Page page, List<Integer> columnValueAndRowIdChannels). The Presto engine calls the updateRows() method of the same UpdatablePageSource instance that supplied the original page, passing a page of projected columns, one for each updated column and the last one for the rowId column. The order of projected columns is defined by the Presto engine, and that order is reflected in the columnValueAndRowIdChannels argument. The job of updateRows() is to:

  • Extract the updated column blocks and the rowId block from the projected page.
  • Assemble them in whatever order is required by the connector for storage.
  • Store the update result in the underlying file store.

• CompletableFuture<Collection<Slice>> finish(). The Presto engine calls finish() when all the pages of a split have been processed. The connector returns a future containing a collection of Slice, representing connector-specific information about the rows processed. Usually this will include the row count, and might include information like the files or partitions created or changed.

ConnectorMetadata DELETE API

A connector implementing DELETE must specify three ConnectorMetadata methods.

• getDeleteRowIdColumnHandle()::

  ColumnHandle getDeleteRowIdColumnHandle( ConnectorSession session, ConnectorTableHandle tableHandle)

  The ColumnHandle returned by this method provides the rowId column handle used by the connector to identify rows to be deleted, as well as any other fields of the row that the connector will need to complete the DELETE operation. For a JDBC connector, that rowId is usually the primary key for the table and no other fields are required. For other connectors, the information needed to identify a row usually consists of multiple physical columns.

• beginDelete()::

  ConnectorDeleteTableHandle beginDelete( ConnectorSession session, ConnectorTableHandle tableHandle)

  As the last step in creating the DELETE execution plan, the connector's beginDelete() method is called, passing the session and tableHandle.

  beginDelete() performs any orchestration needed in the connector to start processing the DELETE. This orchestration varies from connector to connector.

  beginDelete() returns a ConnectorDeleteTableHandle with any added information the connector needs when the handle is passed back to finishDelete() and the split generation machinery. For most connectors, the returned table handle contains a flag identifying the table handle as a table handle for a DELETE operation.

• finishDelete()::

  Copy

  void finishDelete(
      ConnectorSession session,
      ConnectoDeleteTableHandle tableHandle,
      Collection<Slice> fragments)
  

  During DELETE processing, the Presto engine accumulates the Slice collections returned by UpdatablePageSource.finish(). After all splits have been processed, the engine calls finishDelete(), passing the table handle and that collection of Slice fragments. In response, the connector takes appropriate actions to complete the Delete operation. Those actions might include committing the transaction, assuming the connector supports a transaction paradigm.

ConnectorMetadata UPDATE API

A connector implementing UPDATE must specify three ConnectorMetadata methods.

• getUpdateRowIdColumnHandle::

  ColumnHandle getUpdateRowIdColumnHandle( ConnectorSession session, ConnectorTableHandle tableHandle, List<ColumnHandle> updatedColumns)

  The updatedColumns list contains column handles for all columns updated by the UPDATE operation in table column order.

  The ColumnHandle returned by this method provides the rowId used by the connector to identify rows to be updated, as well as any other fields of the row that the connector will need to complete the UPDATE operation.

• beginUpdate::

  ConnectorTableHandle beginUpdate( ConnectorSession session, ConnectorTableHandle tableHandle, List<ColumnHandle> updatedColumns)

  As the last step in creating the UPDATE execution plan, the connector's beginUpdate() method is called, passing arguments that define the UPDATE to the connector. In addition to the session and tableHandle, the arguments includes the list of the updated columns handles, in table column order.

  beginUpdate() performs any orchestration needed in the connector to start processing the UPDATE. This orchestration varies from connector to connector.

  beginUpdate returns a ConnectorTableHandle with any added information the connector needs when the handle is passed back to finishUpdate() and the split generation machinery. For most connectors, the returned table handle contains a flag identifying the table handle as a table handle for a UPDATE operation. For some connectors that support partitioning, the table handle will reflect that partitioning.

• finishUpdate::

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  void finishUpdate(
      ConnectorSession session,
      ConnectorTableHandle tableHandle,
      Collection<Slice> fragments)
  

  During UPDATE processing, the Presto engine accumulates the Slice collections returned by UpdatablePageSource.finish(). After all splits have been processed, the engine calls finishUpdate(), passing the table handle and that collection of Slice fragments. In response, the connector takes appropriate actions to complete the UPDATE operation. Those actions might include committing the transaction, assuming the connector supports a transaction paradigm.