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Commit68ef887

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Remove obsolete nbtree README commentary.
Commitd2086b0 removed almost all cases where nbtree must release aread buffer lock and acquire a write buffer lock instead, so remainingcases in which that's still necessary are not notable enough to appearin the nbtree README.More importantly, holding on to a buffer pin in cases where nbtree musttrade a read lock for a write lock is very unlikely to save any I/O.This seems to have been a long overlooked throwback to a time whennbtree cared about write-ordering dependencies, and performedsynchronous buffer writes. It hasn't worked that way in many years.
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‎src/backend/access/nbtree/README

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@@ -65,10 +65,7 @@ copies of tree pages are unshared. Postgres shares in-memory buffers
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among backends. As a result, we do page-level read locking on btree
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pages in order to guarantee that no record is modified while we are
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examining it. This reduces concurrency but guarantees correct
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behavior. An advantage is that when trading in a read lock for a
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write lock, we need not re-read the page after getting the write lock.
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Since we're also holding a pin on the shared buffer containing the
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page, we know that buffer still contains the page and is up-to-date.
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behavior.
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We support the notion of an ordered "scan" of an index as well as
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insertions, deletions, and simple lookups. A scan in the forward

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