On Mon, Sep 14, 2020 at 6:56 PM Peter Geoghegan <p...@bowt.ie> wrote: > I'm not sure what I was talking about earlier when I connected this > with the main/instrumentation issue, since preallocation used by > logtape.c to help HashAggs-that-spill necessarily reserves blocks > without writing them out for a while (the fires in California have > made it difficult to be productive). You might write blocks out as > zero blocks first, and then only write the real data later > (overwriting the zero blocks). But no matter how the writes among > tapes are interlaced, the fact is that nBlocksAllocated can exceed > nBlocksWritten by at least one block per active tape.
Oh, wait. Of course the point was that we wouldn't even have to use nBlocksAllocated in LogicalTapeSetBlocks() anymore -- we would make the assumption that nBlocksWritten could be used for all callers in all cases. Which is a reasonable assumption once you enforce that there are no active write buffers. Which is evidently a good idea anyway, since it saves on temp file disk space in HashAggs-that-spill/prealloc cases with very little work_mem. -- Peter Geoghegan
