Tom Lane wrote:
> "Pavan Deolasee" <[EMAIL PROTECTED]> writes:
>> Please see the version 14 of HOT patch attached.
> I expected to find either a large new README, or some pretty substantial
> additions to existing README files, to document how this all works.

Here's an updated version of the README I posted earlier. It now
reflects the changes to how pruning works.

  Heikki Linnakangas
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.

The pre-requirements for doing a HOT update is that none of the indexed columns 
are changed. That's 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 is a lightweight vacuum operation that can be run on a single page, 
with no need to scan indexes, but it only removes dead HOT tuples. Other dead 
tuples are truncated, leaving only a redirected dead line pointer. The removed 
tuples are compacted away using PageRepairFragmentation, like in normal vacuum. 
There's two reasons to prune a page: to make room on the page for future 
updates, and to shorten HOT chains to make index lookups cheaper.

When accessing a page with HOT updated tuples on it, and less than a certain 
threshold of free space, we try to prune it. To do that, we need to take a 
vacuum strength lock on the buffer. If that fails, we don't prune; the theory 
is that you usually do get the lock, and if you don't, you'll get to try again 
next time. It would be more logical to do the pruning in heap_update when the 
page is full, but by the time we get there we have already pinned the page and 
have references to tuples on it, so we can't start moving tuples around it. 
Also, that alone wouldn't address the desire to keep HOT chains short, to avoid 
overhead of traversing long chains on index lookups.

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.

When the last live tuple in an update chain becomes dead (after a DELETE or a 
cold update), the redirecting line pointer is marked as redirected dead. That 
allows us to immediately reuse the space, sans the line pointer itself. 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.

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.

There's not much changes to regular vacuum. It removes dead HOT tuples, like 
pruning, and cleans up any redirected dead line pointers.

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.

XXX: How do HOT-updates affect statistics? How often do we need to run 

I'm not very familiar with how these, so I'll just shut up..


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.

        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 
---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
       choose an index scan if your joining column's datatypes do not

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