Fetch Methods for Row Blocks

The fetch methods for row blocks are _fetch_into_extfn and _fetch_block_extfn. These methods are part of the a_v4_extfn_table_func structure.

When producing data, if the table UDF or TPF builds its own row block, the UDF must provide the fetch_block API method. If the UDF does not build its own row block, the UDF must provide the fetch_into API method.

When consuming data, if the TPF builds its own row block, the UDF calls the fetch_into method on the producer. If the TPF does not build its own row block, the TPF must call the fetch_block method on the producer.

The UDF can select which fetch method to use for data production and consumption. In general, these guidelines apply:

• fetch_into – Use this API when the consumer owns the memory for the data transfer area and requests that the producer use this area. In this scenario, the consumer cares about how the data transfer area is set up, and the producer performs the necessary data copies into this area.
• fetch_block – Use this API when the consumer does not care about the format of the data transfer area. fetch_block requests the producer to create a data transfer area and provides a pointer to that area. The consumer owns the memory and the consumer is responsible for copying data from this area.

• The fetch_block Method
  Use the fetch_block method for underlying data storage.
• The fetch_into Method
  Use the fetch_into API when the consumer owns the memory for the data transfer area and requests that the producer use this area.