add explicit partial union-all and improve parallel subquery costing

When Who What
2021-12-02 14:10:32 Daniel Gustafsson (d_gustafsson) Closed in commitfest 2021-11 with status: Returned with feedback
2021-10-06 13:16:36 Jaime Casanova (jcasanov) Closed in commitfest 2021-09 with status: Moved to next CF
2021-08-03 02:27:27 Masahiko Sawada (masahikosawada) Closed in commitfest 2021-07 with status: Moved to next CF
2021-07-23 12:28:56 Ronan Dunklau (rdunklau) Added rdunklau as reviewer
2021-06-21 00:54:14 Greg Nancarrow (gregn1234) Changed authors to Luc Vlaming (luc.vlaming)
2021-04-12 12:03:56 Luc Vlaming (luc.vlaming) Attached mail thread FRXPR01MB09021F98988B057A636AA4EEB7040@FRXPR01MB0902.DEUPRD01.PROD.OUTLOOK.DE
2021-04-12 12:03:56 Luc Vlaming (luc.vlaming) Created patch record