Pavan Deolasee wrote:
Please see updated version of the patch. This includes further code
refactoring and bug fixes.
Thanks for the update, Pavan!
I've been looking at this patch in the last couple of weeks in detail. I
wrote a short summary of how it works (attached) to help reviewing it.
Especially the glossary is helpful, since the patch introduces a lot of
new concepts.
I have some suggestions which I'll post separately, this just describes
the status quo of the patch.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
Use case
--------
The best use case for HOT is a table that's frequently UPDATEd, and is large
enough that VACUUM is painful. On small tables that fit in cache, running
VACUUM every few minutes isn't a problem.
Heap-only tuples
----------------
When a HOT update is performed, the new tuple is placed on the same page as the
old one, marked with the HEAP_ONLY_TUPLE flag. HEAP_ONLY_TUPLE means that
there's no index pointers to the tuple, which allows pruning the chain in the
future. The old tuple is marked with HEAP_HOT_UPDATE-flag, which means that the
tuple pointed to by t_ctid is a heap-only tuple. That needs to be taken into
account when vacuuming, so that we don't remove the root tuple in the update
chain, when there's no index pointers to the later tuples.
When doing an index scan, whenever we reach a non-visible tuple, we need to
check if the tuple has been HOT-updated (== HEAP_HOT_UPDATE flag is set). If
so, we need to follow the ctid pointer until we reach a visible one, or one
that hasn't been HOT-updated.
Sequential scans (and bitmap heap scans with a lossy bitmap) don't need to pay
attention to the flags.
Pre-requirements for HOT updates:
1. None of the indexed columns are changed
2. There is no functional indexes on the table
3. There is no partial indexes on the table
2. and 3. could be relaxed, the code just hasn't been written yet.
These requirements are checked at execution time, comparing the binary
representation of the old and new values. That means that dummy updates, like
"UPDATE foo SET col1 = ?", where ? is the same as the old value can be HOT.
In addition to the above, there needs to be room on the page for the new tuple.
If the page is full, we try to make room by pruning the page.
Pruning
-------
When we're doing a HOT update, and there isn't enough space on the page, and
there's no suitably sized LP_DELETEd tuples to reuse, all HOT update chains on
the page are pruned to make room. Pruning can be thought of as a lightweight
retail vacuum, that marks all dead heap-only tuples with LP_DELETE flag,
allowing them to be reused. We can't just outright remove the tuples like we do
in vacuum, because we'd need a vacuum-strength lock for that.
To reclaim the index-visible (i.e. first) tuple in a HOT chain, the line
pointer is turned into a redirecting line pointer that points to the line
pointer of the next tuple in the chain. To keep track of the space occupied by
the dead tuple, so that we can reuse the space, a new line pointer is allocated
and marked with LP_DELETE to point to the dead tuple. That means its tid
changes, but that's ok since it's dead.
When the last live tuple in an update chain becomes dead (after a DELETE or a
cold update), all tuples in the chain can be marked with LP_DELETE, and the
redirecting line pointer is marked as redirected dead.
We've effectively resurrected the "truncate dead tuples to just line pointer"
idea that has been proposed and rejected before because of fear of line pointer
bloat. To limit the damage in worst case, and to keep numerous arrays as well
as the bitmaps in bitmap scans reasonably sized, the maximum number of line
pointers (MaxHeapTuplesPerPage) is somewhat arbitrarily capped at 2 * what it
was before.
In addition to pruning when a page gets full, pruning of a single HOT chain is
done when doing an index fetch. That avoids doing the same chain-following work
on future fetches of the same row.
VACUUM FULL
-----------
To make vacuum full work, any DEAD tuples in the middle of an update chain
needs to be removed (see comments at the top of heap_prune_hotchain_hard for
details). Vacuum full performs a more aggressive pruning that not only removes
dead tuples at the beginning of an update chain, it scans the whole chain and
removes any intermediate dead tuples as well.
Reusing LP_DELETEd heap tuples
------------------------------
When doing an update, HOT or not, we check if there's a tuple on the page
marked with LP_DELETE that's big enough to accommodate the new tuple. If there
is, that slot is reused, overwriting the deleted tuple.
We could reuse the slots for inserts as well, but as the patch stands, we don't.
Row-level fragmentation
-----------------------
If the new tuple is smaller than the old LP_DELETEd tuple that's reused, the
new tuple is marked as fragmented, which means that there is some unused space
between the end of this tuple and the beginning of the next tuple.
If there's no LP_DELETEd tuples large enough to fit the new tuple in, the
row-level fragmentation is repaired in the hope that some of the slots were
actually big enough, but were just fragmented. That's done by mapping the
offsets in the page, and enlarging all LP_DELETEd line pointers up to the
beginning of the next tuple.
Vacuum
------
Vacuum prunes all HOT chains, and removes any LP_DELETEd tuples, making the
space available for any use.
In lazy vacuum, we must not freeze a tuple that's in the middle of an update
chain. That can happen when a tuple has xmin > xmax; it's the same scenario
that requires "hard pruning" in VACUUM FULL. Freezing such tuples will break
the check that xmin and xmax matches when following the chain. It's not a
problem without HOT, because the preceding tuple in the chain must be dead as
well so no-one will try to follow the chain, but with HOT the preceding tuple
would be DEAD_CHAIN, and someone might still need to follow the chain to find
the live tuple. We avoid that by just not freezing such tuples. They can be
frozen eventually, when the xmax of the preceding tuple is < OldestXmin as well.
Statistics
----------
XXX: How do HOT-updates affect statistics? How often do we need to run
autovacuum?
CREATE INDEX
CREATE INDEX CONCURRENTLY
-------------------------
I'm not very familiar with how these, so I'll just shut up..
Glossary
--------
Fragmented tuple slot
A line pointer with lp_len smaller than the actual space available
before the next tuple on the page.
Heap-only tuple
A heap tuple with no index pointers. Marked with HEAP_ONLY_TUPLE flag.
HOT-updated tuple
An updated tuple, so that the next tuple in the chain is a heap-only
tuple. Marked with HEAP_HOT_UPDATE flag.
Redirecting line pointer
A line pointer that points to another line pointer. lp_len is set to a
magic value (ITEMID_REDIRECTED), and lp_off is the OffsetNumber of the line
pointer it points to.
Redirected dead line pointer
A stub line pointer, that doesn't point to anything, but can't be
removed or reused yet because there is index pointers to it. Semantically same
as a dead tuple.
Root tuple
The first tuple in a HOT update chain, that indexes point to.
Update chain
A chain of updated tuples, so that each tuple's ctid points to the next
tuple in the chain. A HOT update chain is an update chain that consists of a
root tuple and one or more heap-only tuples. An update chain can contain both
HOT and non-HOT (cold) updated tuples.
Cold update
A normal, non-HOT update.
HOT update
An UPDATE, where the new tuple becomes a heap-only-tuple, and no index
entries are made.
DEAD_CHAIN (HEAPTUPLE_DEAD_CHAIN)
New return value for HeapTupleSatisfiesVacuum, which means that the
tuple is not visible to anyone, but it's been HOT updated so we can't remove it
yet because the following tuples in the chain would become inaccessible from
indexes.
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