Fix #18139: correctly initialize flush size in MemoryStream, and re-use writer states - #19398
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Fix duckdb/duckdb#18139: correctly initialize flush size in MemoryStream, and re-use writer states (duckdb/duckdb#19398)
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Fix duckdb/duckdb#18139: correctly initialize flush size in MemoryStream, and re-use writer states (duckdb/duckdb#19398) Co-authored-by: krlmlr <krlmlr@users.noreply.github.com>
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Fixes #18139
The performance variation comes from us triggering a poor case in MacOS' allocator - the issue doesn't occur outside of MacOS and requires a specific number of threads. We can avoid the issue (and it is generally good practice anyway) by doing fewer (re)-allocations.
This was already partially fixed once before in #18174 but that seems to have gotten lost in the refactor of #17692. This PR brings back that fix and adds an additional fix by reducing the number of
CSVWriterStatethat are instantiated during batch writes.