ROUND_ROBIN sets round robin fetch mode, which wraps each child branch of the UNION with a buffered pipe cursor. Each cursor spawns a background thread to prefetch data into its own buffer. When the query is executed, the UNION operator reads from each child pipe cursor in round-robin fashion.

Note: Specifying a fetch mode with SORTMERGE UNION is not usually advisable, because the algorithm reads from both sides.

The <int> argument specifies the maximum number of rows that can be prefetched into each buffer. Optional. The default value is 1000. The maximum value is 2000.