Standard English uses "may", "can", and "might" in different ways:
may - permission, "You may borrow my rake."
can - ability, "I can lift that log."
might - possibility, "It might rain today."
Unfortunately, in conversational English, their use is often mixed, as
in, "You may use this variable to do X", when in fact, "can" is a better
choice. Similarly, "It may crash" is better stated, "It might crash".
in normal operation, and we can avoid rewriting pg_control at every log
segment switch if we don't insist that these values be valid. Reducing
the number of pg_control updates is a good idea for both performance and
reliability. It does make pg_resetxlog's life a bit harder, but that seems
a good tradeoff; and anyway the change to pg_resetxlog amounts to automating
something people formerly needed to do by hand, namely look at the existing
pg_xlog files to make sure the new WAL start point was past them.
In passing, change the wording of xlog.c's "database system was interrupted"
messages: describe the pg_control timestamp as "last known up at" rather than
implying it is the exact time of service interruption. With this change the
timestamp will generally be the time of the last checkpoint, which could be
many minutes before the failure; and we've already seen indications that
people tend to misinterpret the old wording.
initdb forced due to change in pg_control layout. Simon Riggs and Tom Lane
of the transaction ID counter. Nothing is done with the epoch except to
store it in checkpoint records, but this provides a foundation with which
add-on code can pretend that XIDs never wrap around. This is a severely
trimmed and rewritten version of the xxid patch submitted by Marko Kreen.
Per discussion, the epoch counter seems the only part of xxid that really
needs to be in the core server.
transaction IDs, rather than like subtrans; in particular, the information
now survives a database restart. Per previous discussion, this is
essential for PITR log shipping and for 2PC.
to eliminate unnecessary deadlocks. This commit adds SELECT ... FOR SHARE
paralleling SELECT ... FOR UPDATE. The implementation uses a new SLRU
data structure (managed much like pg_subtrans) to represent multiple-
transaction-ID sets. When more than one transaction is holding a shared
lock on a particular row, we create a MultiXactId representing that set
of transactions and store its ID in the row's XMAX. This scheme allows
an effectively unlimited number of row locks, just as we did before,
while not costing any extra overhead except when a shared lock actually
has to be shared. Still TODO: use the regular lock manager to control
the grant order when multiple backends are waiting for a row lock.
Alvaro Herrera and Tom Lane.
be able to do that, but the ability seems to have got lost in the
shuffle). Add a -o nextOID switch for completeness. Improve the
documentation to explain how and why to use these switches.
