b437571714
Allow using multiple worker processes to build BRIN index, which until now was supported only for BTREE indexes. For large tables this often results in significant speedup when the build is CPU-bound. The work is split in a simple way - each worker builds BRIN summaries on a subset of the table, determined by the regular parallel scan used to read the data, and feeds them into a shared tuplesort which sorts them by blkno (start of the range). The leader then reads this sorted stream of ranges, merges duplicates (which may happen if the parallel scan does not align with BRIN pages_per_range), and adds the resulting ranges into the index. The number of duplicate results produced by workers (requiring merging in the leader process) should be fairly small, thanks to how parallel scans assign chunks to workers. The likelihood of duplicate results may increase for higher pages_per_range values, but then there are fewer page ranges in total. In any case, we expect the merging to be much cheaper than summarization, so this should be a win. Most of the parallelism infrastructure is a simplified copy of the code used by BTREE indexes, omitting the parts irrelevant for BRIN indexes (e.g. uniqueness checks). This also introduces a new index AM flag amcanbuildparallel, determining whether to attempt to start parallel workers for the index build. Original patch by me, with reviews and substantial reworks by Matthias van de Meent, certainly enough to make him a co-author. Author: Tomas Vondra, Matthias van de Meent Reviewed-by: Matthias van de Meent Discussion: https://postgr.es/m/c2ee7d69-ce17-43f2-d1a0-9811edbda6e6%40enterprisedb.com
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