Air carrier Captain reported traffic to join parallel final triggered an RA; resulted in a go around and a near midair collision.

Narrative

While flying Aircraft X; ZZZ [to] SFO; we planned and briefed the MLBEC Arrival to be followed by the charted visual for [Runway] 28L. Upon checking in with NORCAL; we where assigned a heading of 100 off of CORKK for the charted visual [Runway] 28R. We changed the LOC frequency; reprogrammed the FMS; and briefed the new approach. We flew a right downwind and were vectored for a right base. ATC pointed out traffic that would merge for [Runway] 28L. We where assigned 160 kts. The ATIS was reporting winds of approximately 260/25G34. With our weight and the wind additive; the REF was 149; target 164. ATC was advised that 164 was our slowest speed. We configured; gear down flaps 15; on base and slowed. ATC gave us another vector to join the LOC course. We had tuned it; but had not planned on using it since they where flying the charted visuals. We armed LOC and continued while configuring for landing. We intercepted the LOC course; and we cleared for the approach. The Aircraft Y was intercepting their LOC for 28L. At about that time; we got a TA from the merging Aircraft Y. I looked to find it visually. It was under our wing turning left; left wing down. At that same time; we got a RA to stop the descent!" I had already had the autopilot and auto-throttle off to hand fly the approach. We started the go around response to the RA. As we where bringing the flaps up; the RA changed to "Climb! Climb now!" We continued with the escape maneuver and climbed to approximately 3;400 MSL. ATC assigned us 3;000 ft. after we finished the RA response. We complied and subsequently flew another approach to an uneventful landing. During the RA response; the RA aircraft symbol showed the Aircraft Y at -100 ft. I visually looked and it was jarring how close the Aircraft Y had come to us. It was under our wing and I can confirm the 100 ft. separation. If I had selected the TCAS to TA only; we could have within probability descended down on top of him; and been involved in a mid air collision."