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Replace polyphase merge algorithm with a simple balanced k-way merge.
The advantage of polyphase merge is that it can reuse the input tapes as output tapes efficiently, but that is irrelevant on modern hardware, when we can easily emulate any number of tape drives. The number of input tapes we can/should use during merging is limited by work_mem, but output tapes that we are not currently writing to only cost a little bit of memory, so there is no need to skimp on them. This makes sorts that need multiple merge passes faster. Discussion: https://www.postgresql.org/message-id/420a0ec7-602c-d406-1e75-1ef7ddc58d83%40iki.fi Reviewed-by: Peter Geoghegan, Zhihong Yu, John Naylor
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