Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
On Friday 10 November 2006 08:53, Simon Riggs wrote:
On Fri, 2006-11-10 at 12:32 +0100, Zeugswetter Andreas ADI SD wrote:
4. although at first it might seem so I see no advantage for vacuum with
overflow
No need to VACUUM the indexes, which is the most expensive part. The
more indexes you have, the more VACUUM costs, not so with HOT.
This isn't exactly true though right? Since the more indexes you have, the
more likely it is that your updating an indexed column, which means HOT isn't
going to work for you. One common use case that seems problematic is the
indexed, frequently updated timestamp field.
--
Robert Treat
Build A Brighter LAMP :: Linux Apache {middleware} PostgreSQL
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Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
On Thu, 2006-11-09 at 18:28 -0500, Tom Lane wrote: Simon Riggs [EMAIL PROTECTED] writes: As more UPDATEs take place these tuple chains would grow, making locating the latest tuple take progressively longer. This is the part that bothers me --- particularly the random-access nature of the search. I wonder whether you couldn't do something involving an initial table fill-factor of less than 50%, and having the first updated version living on the same heap page as its parent. Only when the active chain length was more than one (which you hypothesize is rare) would it actually be necessary to do a random access into the overflow table. Thats appropriate sometimes, not others, but I'll investigate this further so that its possible to take advantage of non-zero fillfactors when they exist. There's a number of distinct use-cases here: If you have a very small, heavily updated table it makes a lot of sense to use lower fillfactors as well. If you have a larger table, using fillfactor 50% immediately doubles the size of the table. If the updates are uneven, as they mostly are because of the Benfold distribution/Pareto principle, then it has been found that leaving space on block doesn't help the heavily updated portions of a table, whereas it hinders the lightly updated portions of a table. TPC-C and TPC-B both have uniformly distributed UPDATEs, so its easy to use the fillfactor to great advantage there. More generally, do we need an overflow table at all, rather than having these overflow tuples living in the same file as the root tuples? As long as there's a bit available to mark a tuple as being this special not-separately-indexed type, you don't need a special location to know what it is. This might break down in the presence of seqscans though. HOT currently attempts to place a subsequent UPDATE on the same page of the overflow relation, but this doesn't happen (yet) for placing multiple versions on same page. IMHO it could, but will think about it. This allows the length of a typical tuple chain to be extremely short in practice. For a single connection issuing a stream of UPDATEs the chain length will no more than 1 at any time. Only if there are no other transactions being held open, which makes this claim a lot weaker. True, but Nikhil has run tests that clearly show HOT outperforming current situation in the case of long running transactions. The need to optimise HeapTupleSatisfiesVacuum() and avoid long chains does still remain a difficulty for both HOT and the current situation. HOT can only work in cases where a tuple does not modify one of the columns defined in an index on the table, and when we do not alter the row length of the tuple. Seems like altering the row length isn't the issue, it's just is there room on the page for the new version. Again, a generous fillfactor would give you more flexibility. The copy-back operation can only work if the tuple fits in the same space as the root tuple. If it doesn't you end up with a tuple permanently in the overflow relation. That might not worry us, I guess. Also, my understanding was that an overwrite operation could not vary the length of a tuple (at least according to code comments). [We'll be able to do that more efficiently when we have plan invalidation] Uh, what's that got to do with it? Currently the HOT code dynamically tests to see if the index columns have been touched. If we had plan invalidation that would be able to be assessed more easily at planning time, in cases where there is no BEFORE trigger. -- Simon Riggs EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
Simon Riggs wrote: On Thu, 2006-11-09 at 18:28 -0500, Tom Lane wrote: Simon Riggs [EMAIL PROTECTED] writes: HOT can only work in cases where a tuple does not modify one of the columns defined in an index on the table, and when we do not alter the row length of the tuple. Seems like altering the row length isn't the issue, it's just is there room on the page for the new version. Again, a generous fillfactor would give you more flexibility. The copy-back operation can only work if the tuple fits in the same space as the root tuple. If it doesn't you end up with a tuple permanently in the overflow relation. That might not worry us, I guess. Also, my understanding was that an overwrite operation could not vary the length of a tuple (at least according to code comments). You can't If someone else has the page pinned, [We'll be able to do that more efficiently when we have plan invalidation] Uh, what's that got to do with it? Currently the HOT code dynamically tests to see if the index columns have been touched. If we had plan invalidation that would be able to be assessed more easily at planning time, in cases where there is no BEFORE trigger. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
Oops, pressed send too early. Ignore the one-line reply I just sent... Simon Riggs wrote: On Thu, 2006-11-09 at 18:28 -0500, Tom Lane wrote: Simon Riggs [EMAIL PROTECTED] writes: HOT can only work in cases where a tuple does not modify one of the columns defined in an index on the table, and when we do not alter the row length of the tuple. Seems like altering the row length isn't the issue, it's just is there room on the page for the new version. Again, a generous fillfactor would give you more flexibility. The copy-back operation can only work if the tuple fits in the same space as the root tuple. If it doesn't you end up with a tuple permanently in the overflow relation. That might not worry us, I guess. You can't move tuples around in a page without holding a Vacuum lock on the page. Some other backend might have the page pinned and have a pointer to a tuple on the page that would get bogus if the tuple is moved. Maybe you could try to get a vacuum lock when doing the update and prereserve space for the new version if you can get one. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
Simon Riggs [EMAIL PROTECTED] writes: Seems like altering the row length isn't the issue, it's just is there room on the page for the new version. Again, a generous fillfactor would give you more flexibility. The copy-back operation can only work if the tuple fits in the same space as the root tuple. If it doesn't you end up with a tuple permanently in the overflow relation. That might not worry us, I guess. I think he's suggesting that you can put the new version in the available space rather than use the space from the existing tuple. You can keep the same line pointer so index entries still refer to the correct tuple. The only problem I see is that if you determine that there's space available when you do the update that space may have disappeared by the table you come along to do the move-back. Perhaps you can do something clever with reserving the space at that time for the later move-back but I fear that'll complicate vacuum and open up risks if the system crashes in that state. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
Hi, This allows the length of a typical tuple chain to be extremely short in practice. For a single connection issuing a stream of UPDATEs the chain length will no more than 1 at any time. Only if there are no other transactions being held open, which makes this claim a lot weaker.True, but Nikhil has run tests that clearly show HOT outperformingcurrent situation in the case of long running transactions. The need to optimise HeapTupleSatisfiesVacuum() and avoid long chains does stillremain a difficulty for both HOT and the current situation.Yes, I carried out some pgbench runs comparing our current HOT update patch with PG82BETA2 sources for the long running transaction case. For an apples to apples comparison we got roughly 170% improvement with the HOT update patch over BETA2. In case of BETA2, since all versions are in the main heap, we end up doing multiple index scans for them. In case of HOT updates, we have a single index entry with the chains getting traversed from the overflow relation. So as Simon has mentioned the need to avoid long chains remains a difficulty for both the situations. Regards, Nikhils -- EnterpriseDB http://www.enterprisedb.com
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
On Fri, 2006-11-10 at 12:32 +0100, Zeugswetter Andreas ADI SD wrote: As more UPDATEs take place these tuple chains would grow, making locating the latest tuple take progressively longer. More generally, do we need an overflow table at all, rather than having these overflow tuples living in the same file as the root tuples? As long as there's a bit available to mark a tuple as being this special not-separately-indexed type, you don't need a special location to know what it is. Yes, I have that same impression. I think this might work, I'll think about it. If you can come up with a test case where we would need this optimisation then I'm sure that can be prototyped. In many cases though an overflow relation is desirable, so ISTM that we might want to be able to do both: have an updated tuple with not-indexed bit set in the heap if on same page, else go to overflow relation. 1. It doubles the IO (original page + hot page), if the new row would have fit into the original page. Yes, but thats a big if, and once it is full that optimisation goes away. Some thoughts: First, it presumes that the second block is not in cache (see later). Second, my observation is that this can happen for some part of the time, but under heavy updates this window of opportunity is not the common case and so this optimisation would not make much difference. However, in some cases, it would be appropriate, so I'll investigate. HOT optimises the case where a tuple in the overflow relation is UPDATEd, so it can place the subsequent tuple version on the same page. So if you perform 100 UPDATEs on a single tuple, the first will write to the overflow relation in a new block, then the further 99 will attempt to write to the same block, if they can. So in many cases we would do only 101 block accesses and no real I/O. 2. locking should be easier if only the original heap page is involved. Yes, but multi-page update already happens now, so HOT is not different on that point. 3. It makes the overflow pages really hot because all concurrent updates compete for the few overflow pages. Which ensures they are in shared_buffers, rather than causing I/O. The way FSM works, it will cause concurrent updaters to spread out their writes to many blocks. So in the case of a single high frequency updater all of the updates go into the same block of the overflow relation, so the optimisation you referred to in (1) does take effect strongly in that case, yet without causing contention with other updaters. The FSM doesn't change with HOT, but the effects of having inserted additional tuples into the main heap are much harder to undo afterwards. The overflow relation varies in size according to the number of updates, not the actual number of tuples, as does the main heap, so VACUUMing will focus on the hotspots and be more efficient, especially since no indexes need be scanned. [Sure we can use heapblock-need-vacuum bitmaps, but there will still be a mix of updated/not-updated tuples in there, so VACUUM would still be less efficient than with HOT]. So VACUUM can operate more frequently on the overflow relation and keep the size reasonable for more of the time, avoiding I/O. Contention is and will remain a HOT topic ;-) I understand your concerns and we should continue to monitor this on the various performance tests that will be run. 4. although at first it might seem so I see no advantage for vacuum with overflow No need to VACUUM the indexes, which is the most expensive part. The more indexes you have, the more VACUUM costs, not so with HOT. 5. the size reduction of heap is imho moot because you trade it for a growing overflow (size reduction only comes from reusing dead tuples and not adding index tuples -- SITC) HOT doesn't claim to reduce the size of the heap. In the presence of a long running transaction, SizeOfHOT(heap + overflow) = SizeOfCurrent(Heap). VACUUM is still required in both cases to manage total heap size. If we have solely UPDATEs and no deletes, then only the overflow relation need be VACUUMed. Could you try to explain the reasoning behind separate overflow storage ? I think the answers above cover the main points, which seem to make the case clearly enough from a design rationale perspective, even without the performance test results to confirm them. What has been stated so far was not really conclusive to me in this regard. Personally, I understand. I argued against them for about a month after I first heard of the idea, but they make sense for me now. HOT has evolved considerably from the various ideas of each of the original idea team (Jonah, Bruce, Jan, myself) and will continue to do so as better ideas replace poor ones, based on performance tests. All of the ideas within it need to be strongly challenged to ensure we arrive at the best solution. e.g. a different header seems no easier in overflow than in heap True. The idea there is that we
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
On 11/10/06, Simon Riggs [EMAIL PROTECTED] wrote: On Fri, 2006-11-10 at 12:32 +0100, Zeugswetter Andreas ADI SD wrote: e.g. a different header seems no easier in overflow than in heap True. The idea there is that we can turn frequent update on/off fairlyeasily for normal tables since there are no tuple format changes in themain heap. It also allows HOT to avoid wasting space when a table is heavily updated in certain places only.I general though, it would make implementation a bit simpler when tuples withdifferent header are isolated in a different relation. Regards, Pavan
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
Hi, True, but Nikhil has run tests that clearly show HOT outperforming current situation in the case of long running transactions. The need to optimise HeapTupleSatisfiesVacuum() and avoid long chains does still remain a difficulty for both HOT and the current situation. Yes, I carried out some pgbench runs comparing our current HOT update patch with PG82BETA2 sources for the long running transaction case. For an apples to apples comparison we got roughly 170% improvement with the HOT update patch over BETA2. In case of BETA2, since all versions are in the main heap, we end up doing multiple index scans for them. In case of HOT updates, we have a single index entry with the chains getting traversed from the overflow relation. So as Simon has mentioned the need to avoid long chains remains a difficulty for both the situations. Regards, Nikhils ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
2. locking should be easier if only the original heap page is involved. Yes, but multi-page update already happens now, so HOT is not different on that point. I was thinking about the case when you pull back a tuple, which seems to be more difficult than what we have now. Andreas PS: I think it is great that you are doing all this work and explaining it for us. Thanks. ---(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 match
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
True, but Nikhil has run tests that clearly show HOT outperforming current situation in the case of long running transactions. The need to optimise HeapTupleSatisfiesVacuum() and avoid long chains does still remain a difficulty for both HOT and the current situation. Yes, I carried out some pgbench runs comparing our current HOT update patch with PG82BETA2 sources for the long running transaction case. For an apples to apples comparison we got Vaccuums every 5 minutes, or no vaccuums ? roughly 170% improvement with the HOT update patch over BETA2. Wow, must be smaller indexes and generally less index maintenance. What this also states imho, is that following tuple chains is not so expensive as maintaining indexes (at least in a heavy update scenario like pgbench). Maybe we should try a version, where the only difference to now is, that when the index keys stay the same the indexes are not updated, and the tuple chain is followed instead when selecting with index. (Maybe like the current alive flag the index pointer can even be refreshed to the oldest visible tuple by readers) Andreas ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
Hi, On 11/10/06, Zeugswetter Andreas ADI SD [EMAIL PROTECTED] wrote: True, but Nikhil has run tests that clearly show HOT outperforming current situation in the case of long running transactions. The need to optimise HeapTupleSatisfiesVacuum() and avoid long chains does still remain a difficulty for both HOT and the current situation. Yes, I carried out some pgbench runs comparing our current HOT update patch with PG82BETA2 sources for the long running transaction case. For an apples to apples comparison we gotVaccuums every 5 minutes, or no vaccuums ? We tried with both. Vacuumseems to dolittle to help in a long running transaction case. Generally in most of the pgbench runs that we carried out, autovacuum did not seem to be of much help even to PG82. Regards, Nikhils-- EnterpriseDB http://www.enterprisedb.com
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
On Fri, 2006-11-10 at 16:46 +0100, Zeugswetter Andreas ADI SD wrote: I'm not sure this really solves that problem because there are still DELETEs to consider but it does remove one factor that exacerbates it unnecessarily. Yea, so you still need to vaccum the large table regularly. HOT covers the use-case of heavy updating, which in many common cases occurs on tables with few inserts/deletes. HOT would significantly reduce the need to vacuum since deletes and wraparound issues would be the only remaining reasons to do this. [I have some ideas for how to optimize tables with heavy INSERT/DELETE activity, but that case is much less prevalent than heavy UPDATEs.] I think the vision is that the overflow table would never be very large because it can be vacuumed very aggressively. It has only tuples that are busy and will need vacuuming as soon as a transaction ends. Unlike the main table which is mostly tuples that don't need vacuuming. Ok, but you have to provide an extra vacuum that does only that then (and it randomly touches heap pages, and only does partial work there). Sure, HOT needs a specially optimised VACUUM. So a heap that's double in size necessary takes twice as long as necessary to scan. The fact that the overflow tables are taking up space isn't interesting if they don't have to be scanned. The overflow does have to be read for each seq scan. And it was stated that it would be accessed with random access (follow tuple chain). But maybe we can read the overflow same as if it where an additional segment file ? Not without taking a write-avoiding lock on the table, unfortunately. Hitting the overflow tables should be quite rare, it only comes into play when looking at concurrently updated tuples. It certainly happens but most tuples in the table will be committed and not being concurrently updated by anyone else. The first update moves the row to overflow, only the 2nd next might be able to pull it back. So on average you would have at least 66% of all updated rows after last vacuum in the overflow. The problem with needing very frequent vacuums is, that you might not be able to do any work because of long transactions. HOT doesn't need more frequent VACUUMs, it is just more efficient and so can allow them, when needed to avoid I/O. Space usage in the overflow relation is at its worst in the case of an enormous table with low volume random updates, but note that it is *never* worse than current space usage. In the best case, which is actually fairly common in practice: a small number of rows of a large table are being updated by a steady stream of concurrent updates, we find the overflow relation needs only a few 100 tuples, so regular vacuuming will be both easy and effective. As an aside, note that HOT works best in real-world situations, not benchmarks such as TPC where the I/Os are deliberately randomised to test the scalability of the RDBMS. But even then, HOT works better. The long-running transaction issue remains unsolved in this proposal, but I have some ideas for later. -- Simon Riggs EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
On Thu, 2006-11-09 at 18:49 +0100, Martijn van Oosterhout wrote: Nice idea, just one question: It seems to me that bitmap index scans will get these same characteristics also, right? The bitmap scan will have to follow the chain of any possibly matching tuple in any of the blocks that are in the bitmap, right? Yes, they would identify the root tuples. The whole chain has matching index values, by definition, so the re-evaluation for lossy bitmaps will work just the same before the actual tuple is retrieved by walking the chain (if required). -- Simon Riggs EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
Re: [HACKERS] Frequent Update Project: Design Overview of HOTUpdates
On Thu, 2006-11-09 at 13:21 -0800, Josh Berkus wrote: Simon, If we perform an update that meets the HOT criteria then we put the new version into the overflow relation; we describe this as a HOT UPDATE. If we perform an update that does not meet the criteria, then we carry on with the existing/old MVCC behaviour; we describe this as a non-HOT UPDATE. Making the essential performance analysis question, Am I HOT or Not? Very good. ;-) Well, we had Overflow Update CHaining as an alternative name... :-) The naming sounds silly, but we had a few alternate designs, so we needed to be able to tell them apart sensibly. We've had TVR, SITC, UIP and now HOT. Software research... -- Simon Riggs EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 6: explain analyze is your friend
