In a hash-based table scan on an allpages-locked table, multiple worker processes scan a single chain of data pages in a table simultaneously. All worker processes traverse the page chain and apply an internal hash function to each page ID. The hash function determines which worker process reads the rows in the current page. The hash function ensures that only one worker process scans the rows on any given page of the table. Figure 25-3 illustrates the hash-based table scan.
Figure 25-3: Parallel hash-based table scan on an allpages-locked table
The hash-based scan provides a way to distribute the processing of a single chain of data pages over multiple engines. The optimizer may use this access method for the outer table of a join query to process a join condition in parallel.
