Improve documentation about MVCC-unsafe utility commands.
The table-rewriting forms of ALTER TABLE are MVCC-unsafe, in much the same
way as TRUNCATE, because they replace all rows of the table with newly-made
rows with a new xmin. (Ideally, concurrent transactions with old snapshots
would continue to see the old table contents, but the data is not there
anymore --- and if it were there, it would be inconsistent with the table's
updated rowtype, so there would be serious implementation problems to fix.)
This was nowhere documented though, and the problem was only documented for
TRUNCATE in a note in the TRUNCATE reference page. Create a new "Caveats"
section in the MVCC chapter that can be home to this and other limitations
on serializable consistency.
In passing, fix a mistaken statement that VACUUM and CLUSTER would reclaim
space occupied by a dropped column. They don't reconstruct existing tuples
so they couldn't do that.
Back-patch to all supported branches.
Branch
------
REL9_0_STABLE
Details
-------
http://git.postgresql.org/pg/commitdiff/ec2bc39e8910d05e27794e5095b7b8daed981406
Modified Files
--------------
doc/src/sgml/mvcc.sgml | 20 ++++++++++++++++++++
doc/src/sgml/ref/alter_table.sgml | 10 +++++++++-
doc/src/sgml/ref/truncate.sgml | 23 ++++++-----------------
3 files changed, 35 insertions(+), 18 deletions(-)
