$Header: /cvsroot/pgsql/src/backend/access/nbtree/README,v 1.1.1.1 1996/07/09 06:21:12 scrappy Exp $
This directory contains a correct implementation of Lehman and Yao's
btree management algorithm that supports concurrent access for Postgres.
We have made the following changes in order to incorporate their algorithm
into Postgres:
+ The requirement that all btree keys be unique is too onerous,
but the algorithm won't work correctly without it. As a result,
this implementation adds an OID (guaranteed to be unique) to
every key in the index. This guarantees uniqueness within a set
of duplicates. Space overhead is four bytes.
For this reason, when we're passed an index tuple to store by the
common access method code, we allocate a larger one and copy the
supplied tuple into it. No Postgres code outside of the btree
access method knows about this xid or sequence number.
+ Lehman and Yao don't require read locks, but assume that in-
memory copies of tree nodes are unshared. Postgres shares
in-memory buffers among backends. As a result, we do page-
level read locking on btree nodes in order to guarantee that
no record is modified while we are examining it. This reduces
concurrency but guaranteees correct behavior.
+ Read locks on a page are held for as long as a scan has a pointer
to the page. However, locks are always surrendered before the
sibling page lock is acquired (for readers), so we remain deadlock-
free. I will do a formal proof if I get bored anytime soon.
In addition, the following things are handy to know:
+ Page zero of every btree is a meta-data page. This page stores
the location of the root page, a pointer to a list of free
pages, and other stuff that's handy to know.
+ This algorithm doesn't really work, since it requires ordered
writes, and UNIX doesn't support ordered writes.
+ There's one other case where we may screw up in this
implementation. When we start a scan, we descend the tree
to the key nearest the one in the qual, and once we get there,
position ourselves correctly for the qual type (eg, <, >=, etc).
If we happen to step off a page, decide we want to get back to
it, and fetch the page again, and if some bad person has split
the page and moved the last tuple we saw off of it, then the
code complains about botched concurrency in an elog(WARN, ...)
and gives up the ghost. This is the ONLY violation of Lehman
and Yao's guarantee of correct behavior that I am aware of in
this code.
Notes to operator class implementors:
With this implementation, we require the user to supply us with
a procedure for pg_amproc. This procedure should take two keys
A and B and return < 0, 0, or > 0 if A < B, A = B, or A > B,
respectively. See the contents of that relation for the btree
access method for some samples.
Notes to mao for implementation document:
On deletions, we need to adjust the position of active scans on
the index. The code in nbtscan.c handles this. We don't need to
do this for splits because of the way splits are handled; if they
happen behind us, we'll automatically go to the next page, and if
they happen in front of us, we're not affected by them. For
insertions, if we inserted a tuple behind the current scan location
on the current scan page, we move one space ahead.
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