Обсуждение: Re: [HACKERS] Fwd: Joins and links
Leon <leon@udmnet.ru> wrote:
> Earlier I proposed that links should be of type similar to tid,
> so inserts should be fed with values of tid. But this requires
> intermediate step, so there can be a function which takes primary
> key and returns tid, or as you say a function
> last_touched('other_table_name') - this seems the best choice.
Beware of adding special purpose hard-links as a way to
skip the run-time value comparisons. A link looks attractive
but it really only works for one-to-one relationships
(any multi-way relationships would require a list of links
to follow) and a link has all of the overhead that a
foreign key requires.
As somone who has developed several commercial dbms systems,
I would discourage doing a special "link" type. There are
other ways to gain performance -- de-normalize your tables
if you are doing mainly reads; carefully check your storage
layout; and, of course, buy more RAM ;-)
--
Bob Devine devine@cs.utah.edu
> Leon <leon@udmnet.ru> wrote:
> > Earlier I proposed that links should be of type similar to tid,
> > so inserts should be fed with values of tid. But this requires
> > intermediate step, so there can be a function which takes primary
> > key and returns tid, or as you say a function
> > last_touched('other_table_name') - this seems the best choice.
>
> Beware of adding special purpose hard-links as a way to
> skip the run-time value comparisons. A link looks attractive
> but it really only works for one-to-one relationships
> (any multi-way relationships would require a list of links
> to follow) and a link has all of the overhead that a
> foreign key requires.
>
> As somone who has developed several commercial dbms systems,
> I would discourage doing a special "link" type. There are
> other ways to gain performance -- de-normalize your tables
> if you are doing mainly reads; carefully check your storage
> layout; and, of course, buy more RAM ;-)
Good to see you around Bob. This guy does know what he is talking
about.
-- Bruce Momjian | http://www.op.net/~candle maillist@candle.pha.pa.us | (610)
853-3000+ If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill,
Pennsylvania19026
Bob Devine wrote: > Beware of adding special purpose hard-links as a way to > skip the run-time value comparisons. A link looks attractive > but it really only works for one-to-one relationships > (any multi-way relationships would require a list of links > to follow) Not exactly. If you have a fixed set of links it a tuple, you don't have to follow the list of them. > and a link has all of the overhead that a > foreign key requires. > We looked at the matter carefully and found no overhead like foregn key's. Maybe you should read the thread more carefully once again. > As somone who has developed several commercial dbms systems, > I would discourage doing a special "link" type. There are > other ways to gain performance -- de-normalize your tables > if you are doing mainly reads; If I denormalize my tables, they will grow some five to ten times in size. But simply think what you are proposing: you are proposing exactly to break RDBMS "alphabet" to gain performance! This means that even SQL warriors see RDBMS's ideology as not proper and as corrupt, because it hinders performance. You are contradicting yourself! > carefully check your storage > layout; and, of course, buy more RAM ;-) And what will I do with performance loss from bloated tables? -- Leon.
Bruce Momjian wrote: > > > > As somone who has developed several commercial dbms systems, > > I would discourage doing a special "link" type. There are > > other ways to gain performance -- de-normalize your tables > > if you are doing mainly reads; carefully check your storage > > layout; and, of course, buy more RAM ;-) > > Good to see you around Bob. This guy does know what he is talking > about. Believe me, I know what I say. Some day I spoke exactly like you, but having seen an impementation of network DBMS, I suddenly realized that SQL days are numbered. The sooner you understand that the better. -- Leon.
Bob Devine wrote: > As somone who has developed several commercial dbms systems, > I would discourage doing a special "link" type. Of course you tried to implement links and failed, didn't you? It such case personally I and maybe others want to hear what can go wrong, in order to benefit from your mistakes and lessons. -- Leon.
Bruce Momjian wrote: > > As somone who has developed several commercial dbms systems, > > I would discourage doing a special "link" type. There are > > other ways to gain performance -- de-normalize your tables > > if you are doing mainly reads; carefully check your storage > > layout; and, of course, buy more RAM ;-) > > Good to see you around Bob. This guy does know what he is talking > about. > After thinking a bit, it became clear to me that we are flaming senselessly here. So can anyone do a fast hack to test links for speed? Especially with three or more tables being joined. -- Leon.
Leon wrote: > If I denormalize my tables, they will grow some five to ten > times in size. > > But simply think what you are proposing: you are proposing > exactly to break RDBMS "alphabet" to gain performance! This > means that even SQL warriors see RDBMS's ideology as not > proper and as corrupt, because it hinders performance. and he wrote: > After thinking a bit, it became clear to me that we are flaming > senselessly here. So can anyone do a fast hack to test links > for speed? Especially with three or more tables being joined. and in another message: > Of course you tried to implement links and failed, didn't you? > It such case personally I and maybe others want to hear what > can go wrong, in order to benefit from your mistakes and lessons. It's a good idea to test it out. My guess is that a hard link between tables would speed up the join a small amount. The bigger drawbacks are: 1) the application design is now encoded in the database structure. Using link forces your _one_ application's need to affect all other users of that table. Each affected table would be bloated with at least one more column. All updates now affect multiple tables leading to more locking, paging, and synchronization overhead. Etc. 2) adding performance tweaks for a version condemns you to always be aware of it for future versions. I know of many cases where people now hate the idea of a database performance "improvement" that has prevented them from modifying the database schema. One person's company is still using a database that everyone hates because one critical application prevents them from changing it. Indexes are about the only useful physical level hack that have survived the test of time. An index is not part of relational databases but are universally implemented because they yield a huge payback. 3) Be aware of hardware improvements. System performance is still doubling every 18 months. If a software hack can't match that rate, it is probably not worth doing. In my experience, old style network and hierarchical databases are still faster than relational systems. Just like OO DBMSs can be faster. However, the non-relational databases gain their speed by optimizing for a single application instead of being a more general purpose approach. Nearly every company that uses databases realizes that flexibility is more important than a bit more speed unless that business has already maxed out their computer's performance and are desparate for that extract bit. It is my many years of watching databases in use that suggest that links are not worth the overhead. My gut feeling is that links would speed up a simple join by only 10% and there are many other ways to speed up joins. -- Bob Devine devine@cs.utah.edu
[Charset koi8-r unsupported, filtering to ASCII...] > Bruce Momjian wrote: > > > > As somone who has developed several commercial dbms systems, > > > I would discourage doing a special "link" type. There are > > > other ways to gain performance -- de-normalize your tables > > > if you are doing mainly reads; carefully check your storage > > > layout; and, of course, buy more RAM ;-) > > > > Good to see you around Bob. This guy does know what he is talking > > about. No. I wasn't flaming, just confirming that he has lots of experience. -- Bruce Momjian | http://www.op.net/~candle maillist@candle.pha.pa.us | (610) 853-3000+ If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania19026
Bob Devine wrote: > The bigger drawbacks are: > 1) the application design is now encoded in the database structure. This is true. > Using link forces your _one_ application's need to affect all other > users of that table. Each affected table would be bloated with > at least one more column. In fact link is intended to replace foreign key in a given table and not coexist with it. Given that it eliminates the need of index, there is even a small space gain. > All updates now affect multiple tables > leading to more locking, paging, and synchronization overhead. Etc. Oh, no :) After a short discussion it became clear that there must not be a link rewrite in a referencing table during update. So update goes as usual, involving only one table. Instead we have a chain of referenced tuples left after update. VACUUM eliminates these. > > 2) adding performance tweaks for a version condemns you to always > be aware of it for future versions. Absolutely right. If we started a talk on general matters, let me clear my position. Every tool is suitable for it's purpose. No one walks from city to city and uses car instead. And no one takes a car to get into neighbor's home for evening tea :) So. There are tasks of different kind. Some are flexible and require redesigning of relationships often. But there are other, which are well known and explored well, and have well known structure. Accounting is some of them. There are a lot others, without doubt. What is proposed is a tool to handle tasks of the second sort effectively, since general RDBMS is a tool for other, flexible tasks. This is a matter of design and designer's job to choose the right tool. If designer made a wrong choice, it is a problem of him an his kicked ass. You should give designer as many tools as possible and let him choose. They will love you for that :) > 3) Be aware of hardware improvements. System performance is > still doubling every 18 months. If a software hack can't match > that rate, it is probably not worth doing. Oh, that argument again :) I'll tell you - sooner or later this development will stop. There are purely physical obstacles that prevent manufacturing of silicon chips with frequencies much higher than 10 gigahertz. > It is my many years of watching databases in use that suggest > that links are not worth the overhead. My gut feeling is that > links would speed up a simple join by only 10% and there are > many other ways to speed up joins. Let's count. We have two tables, joined by link. What is the cost of lookup? First there is an index scan, which is between 2 and 5 iterations, and link lookup which is 1 iteration. Average is 4 iterations. And if we don't have link, there is 6 iterations. More than 10% already! We still didn't consider joining multiple tables and big tables. So the gain will be big anyway. That is not to consider the optimizer (do I sound like a broken record? :) To be sincere, current Postgres optimizer sucks heavily and in most cases can't figure out the fastest way. Implementing links is a quick and cheap way to get a performance gain on a wide range of tasks. I am obliged to repeat this again and again, because every day there appears a new developer who didn't hear that yet :) -- Leon.
Leon <leon@udmnet.ru> writes: > > 3) Be aware of hardware improvements. System performance is > > still doubling every 18 months. If a software hack can't match > > that rate, it is probably not worth doing. > > Oh, that argument again :) I'll tell you - sooner or later > this development will stop. There are purely physical obstacles > that prevent manufacturing of silicon chips with frequencies much > higher than 10 gigahertz. Furthermore, the continuous availability of ever faster hardware at low prices will slow down very soon, now that the MS Windows users finally don't need to upgrade to twice as fast computers every 18 months just to be able to run the latest version of MS bloatware, and will spend their money on peripherals and fast net access instead. ...but as for "purely physical obstacles", I don't buy it. We will always find a way to make what we need. Count on it. -tih -- Popularity is the hallmark of mediocrity. --Niles Crane, "Frasier"
Bob Devine <devine@cs.utah.edu> writes: > 3) Be aware of hardware improvements. System performance is > still doubling every 18 months. If a software hack can't match > that rate, it is probably not worth doing. I like Kernighan's and Pike's argument, presented in their recent book, The Practice of Programming, that if a software improvement is expected to save more accumulated user time than the programmer time spent making it, then it should be considered worthwhile. Great book, by the way. Highly recommended. -tih -- Popularity is the hallmark of mediocrity. --Niles Crane, "Frasier"
Leon wrote: > > Bob Devine wrote: > > > It is my many years of watching databases in use that suggest > > that links are not worth the overhead. My gut feeling is that > > links would speed up a simple join by only 10% and there are > > many other ways to speed up joins. > > Let's count. We have two tables, joined by link. What is the > cost of lookup? First there is an index scan, which is between > 2 and 5 iterations, and link lookup which is 1 iteration. Average > is 4 iterations. This is true for the case wher you want to look up only one row. The difference will quickly degrade as more rows are fetched in one query and cache misses and disk head movement start rattling your disks. The analogy being a man who needs 10 different items from a supermarket and takes 10 full round trips from home to buy them. > And if we don't have link, there is 6 iterations. > More than 10% already! We still didn't consider joining multiple > tables and big tables. I think that the two-tables-one-row lookup will gain the most, probably even more than 10% > So the gain will be big anyway. > > That is not to consider the optimizer (do I sound like a broken > record? :) To be sincere, current Postgres optimizer sucks heavily > and in most cases can't figure out the fastest way. Adding links does nothing to improve the optimizer, its still free to choose sucky plans. It is possible that links are faster if used in the right way, as they cut out the index lookup, but I suspect that hard-coding link-is-always-faster into the optimiser would also produce a lot of very bad plans. The link-is-always-faster is probably true only for all-memory databases, and even there not allways - for example if it happened to produce a worse initial ordering for sort/group by than some other strategy, a complex query can still run slower (the difference will be small either way) > Implementing links is a quick and cheap way to get a performance > gain on a wide range of tasks. Fixing the optimizer would get a performance gain on a far wider range of tasks, and is still needed for links. > I am obliged to repeat this again and again, > because every day there appears a new developer who didn't hear > that yet :) Unfortunaltely there are far less _developers_ than letter-writers, and it is sometimes quite hard to make them even commit good and useful patches that are ready. So I quess thet if you want links in foreseeable future, your best bet would be to start coding, and to coordinate with whoever starts to fix/rewrite the optimizer (probably Vadim) (BTW, in PostgreSQL, I still consider myself a letter-writer and not developer, as I have committed no code for the backend) ------------- Hannu
Hannu Krosing wrote: > The difference will quickly degrade as more rows are fetched in one > query and cache misses and disk head movement start rattling your > disks. The analogy being a man who needs 10 different items from a > supermarket and takes 10 full round trips from home to buy them. Frankly, I didn't even consider fetching database from disk. This slows queries immensely and I wonder if there exist someone who doesn't keep their entire DB in RAM. > I think that the two-tables-one-row lookup will gain the most, > probably even more than 10% I think the gain will raise with the number of tables, because the more tables - the more index lookups are saved. > Adding links does nothing to improve the optimizer, its still free > to choose sucky plans. It is possible that links are faster if used > in the right way, as they cut out the index lookup, but I suspect that > hard-coding link-is-always-faster into the optimiser would also produce > a lot of very bad plans. Methinks that hard-wiring link-is-always-faster into optimizer will still help it very much, because there are few situations where it is not true. > Fixing the optimizer would get a performance gain on a far wider > range of tasks, and is still needed for links. But general fixing of optimizer is total rewritement of it, whereas link fix is almost a fast hack. > So I quess thet if you want links in foreseeable future, your best bet > would be to start coding, and to coordinate with whoever starts to > fix/rewrite > the optimizer (probably Vadim) > Unfortunately I already have a project to work on. There is too little of me for two projects. -- Leon.
Leon wrote: > > Hannu Krosing wrote: > > > The difference will quickly degrade as more rows are fetched in one > > query and cache misses and disk head movement start rattling your > > disks. The analogy being a man who needs 10 different items from a > > supermarket and takes 10 full round trips from home to buy them. > > Frankly, I didn't even consider fetching database from disk. This > slows queries immensely and I wonder if there exist someone who > doesn't keep their entire DB in RAM. Well, I personally dont even know, how I could keep my entire PostgreSQL DB in RAM :) It would be interesting to know what percentage of people do use PostgreSQL for databases that are small enough to fit in RAM - surely not the ones who need splitting of tables >2GB. And I think that setting up PostgreSQL for maximum RAM usage would make a nice topic for "Optimizing PostgreSQL". When my backends are mostly idle they usually use about 3-4MB of memory, (hardly enough for any database :). It is quite possible that some bigger tables end up in a disk-cache, but you can expect to find all your data in that cache only if you do many queries on the same tables in a row, and the machine is otherways idle. > I think the gain will raise with the number of tables, because > the more tables - the more index lookups are saved. My point is that sometimes even sequential scan is faster than index lookup, and not only due to overhead of using the index, but due to better disk performance of sequential reads vs. random reads For in-memory databases this of course does not count. Still I'm quite sure that the main effort in PostgreSQL development has so far gone to optimising queries where most of the data is fetched from the disk. > > Fixing the optimizer would get a performance gain on a far wider > > range of tasks, and is still needed for links. > > But general fixing of optimizer is total rewritement of it, whereas > link fix is almost a fast hack. I'm not too sure about it. It certainly can be done without a _total_ rewrite, but getting all the new node types and access methods into the parser/planner/executor may not be trivial. One idea would be a cross-table OID index for anything in memory. Then, assuming that everything is in-memory, using oids as links would be only trivially, if at all, slower (2-10 memory accesses and comparisons) than "straight" link lookup, that could also be chasing linear chains of forward-id-links on frequently updated DBs. On infrequently updated DBs you could just use triggers and/or cron jobs to keep your reports updated, I quess that this is what most commercial OLAP systems do. Actually I lived my first halfyear of using PostgreSQL under a delusion that lookup by OID would be somewhat special (fast). Probably due to my misunderstanding of the (ever diminishing) O in ORDBMS :) There have been some attempts to get the object-orientedness better supported by PGSQL, (possibly even some contrib funtions), but nobody seems to have needed it bad enough to 1) implement itand 2) shout long enough to get it included in standart distibution. Most (all?) of the developers seem to be die-hard RDBMS guys and thanks to that we have now a solid and reasonably fast Relational DBMS with some OO rudiments So I quess that unless you do (at least part of) links, no-one else will ;( > Unfortunately I already have a project to work on. There is too > little of me for two projects. Darn! I almost hoped we would get one more PostgreSQL hacker as I'm sure that after familiarising oneself with the code enougth to implement links one would be quite capable of helping with most of PostgreSQL development <grin> ---------------------- Hannu
Hello Hannu, Friday, July 09, 1999 you wrote: H> Still I'm quite sure that the main effort in PostgreSQL development has H> so H> far gone to optimising queries where most of the data is fetched from H> the H> disk. Oh, I see. This is appropriate for some not time critical and dumb applications, such as web DB. But this is out of the way of speed server tasks. Maybe Postgres has been designed with such plan in mind - to use big DBs from disc? That is not good news for me either. Almost everyone has suggested me to use more RAM to speed up queries, and now it turned out to be not in Postgres's mainstream. Maybe there is something wrong with this ideology, since RAM is bigger and cheaper every day? H> forward-id-links on frequently updated DBs. On infrequently updated DBs H> you could just use triggers and/or cron jobs to keep your reports H> updated, H> I quess that this is what most commercial OLAP systems do. It seems that trigger will be the last resort. Best regards, Leon
> Unfortunaltely there are far less _developers_ than letter-writers, and > it > is sometimes quite hard to make them even commit good and useful patches > that are ready. > > So I quess thet if you want links in foreseeable future, your best bet > would be to start coding, and to coordinate with whoever starts to > fix/rewrite > the optimizer (probably Vadim) Are people complaining about the 6.5 optimizer, or the pre-6.5 optimizer? 6.5 has a much improved optimizer. -- Bruce Momjian | http://www.op.net/~candle maillist@candle.pha.pa.us | (610) 853-3000+ If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania19026
Hannu Krosing wrote: > Leon wrote: > > Frankly, I didn't even consider fetching database from disk. This > > slows queries immensely and I wonder if there exist someone who > > doesn't keep their entire DB in RAM. > > Well, I personally dont even know, how I could keep my entire PostgreSQL > DB in RAM :) I thought about doing this once on a Linux box. What I was thinking about was creating a large RAM disk, and use that disk together with a physical drive in a mirror setup. However, I was never able to create a large enough RAM disk back then (must have been like LInux 2.0), and also the RAID mirror code wasn't able to support such a mix of devices (i.e. RAM disk + physical disk). The situation might have changed by now. Maarten -- Maarten Boekhold, boekhold@tibco.com TIBCO Finance Technology Inc. The Atrium Strawinskylaan 3051 1077 ZX Amsterdam, The Netherlands tel: +31 20 3012158, fax: +31 20 3012358 http://www.tibco.com
Then <boekhold@tibco.com> spoke up and said: > Hannu Krosing wrote: > > Leon wrote: > > > Frankly, I didn't even consider fetching database from disk. This > > > slows queries immensely and I wonder if there exist someone who > > > doesn't keep their entire DB in RAM. > > > > Well, I personally dont even know, how I could keep my entire PostgreSQL > > DB in RAM :) > > I thought about doing this once on a Linux box. What I was thinking about was > creating a large RAM disk, and use that disk together with a physical drive in > a mirror setup. However, I was never able to create a large enough RAM disk back then > (must have been like LInux 2.0), and also the RAID mirror code wasn't able to > support such a mix of devices (i.e. RAM disk + physical disk). The situation might > have changed by now. Maarten, PostgreSQL keeps it's data in the filesystem, rather than on raw disks. Due to the nature of *nix, all you need to do to keep your entire DB in memory is have enough memory. The buffer cache will do the rest, for you. Of course, you still need to start it up with -F to avoid fsync's. This is also somewhat OS dependent, as you may have to do some tuning to allow full memory utilization in this manner. -- ===================================================================== | JAVA must have been developed in the wilds of West Virginia. | | After all, why else would it support only single inheritance?? | ===================================================================== | Finger geek@cmu.edu for my public key. | =====================================================================
> > > Well, I personally dont even know, how I could keep my entire PostgreSQL > > > DB in RAM :) > > > > I thought about doing this once on a Linux box. What I was thinking about was > > creating a large RAM disk, and use that disk together with a physical drive in > > a mirror setup. However, I was never able to create a large enough RAM disk back then [...] > Maarten, PostgreSQL keeps it's data in the filesystem, rather than on > raw disks. Due to the nature of *nix, all you need to do to keep your > entire DB in memory is have enough memory. The buffer cache will do > the rest, for you. Of course, you still need to start it up with -F I know, but there's no *guarantee* that the complete database is going to be in RAM. That's what I was trying to solve. Putting the thing on a RAM disk would guarantee that it is. Maarten -- Maarten Boekhold, boekhold@tibco.com TIBCO Finance Technology Inc. The Atrium Strawinskylaan 3051 1077 ZX Amsterdam, The Netherlands tel: +31 20 3012158, fax: +31 20 3012358 http://www.tibco.com
Bob Devine wrote: > Beware of adding special purpose hard-links as a way to > skip the run-time value comparisons. A link looks attractive > but it really only works for one-to-one relationships > (any multi-way relationships would require a list of links > to follow) Not exactly. If you have a fixed set of links it a tuple, you don't have to follow the list of them. > and a link has all of the overhead that a > foreign key requires. > We looked at the matter carefully and found no overhead like foregn key's. Maybe you should read the thread more carefully once again. > As somone who has developed several commercial dbms systems, > I would discourage doing a special "link" type. There are > other ways to gain performance -- de-normalize your tables > if you are doing mainly reads; If I denormalize my tables, they will grow some five to ten times in size. But simply think what you are proposing: you are proposing exactly to break RDBMS "alphabet" to gain performance! This means that even SQL warriors see RDBMS's ideology as not proper and as corrupt, because it hinders performance. You are contradicting yourself! > carefully check your storage > layout; and, of course, buy more RAM ;-) And what will I do with performance loss from bloated tables? -- Leon.
Bob Devine <devine@cs.utah.edu> writes: > 3) Be aware of hardware improvements. System performance is > still doubling every 18 months. If a software hack can't match > that rate, it is probably not worth doing. I like Kernighan's and Pike's argument, presented in their recent book, The Practice of Programming, that if a software improvement is expected to save more accumulated user time than the programmer time spent making it, then it should be considered worthwhile. Great book, by the way. Highly recommended. -tih -- Popularity is the hallmark of mediocrity. --Niles Crane, "Frasier"
Hannu Krosing wrote: > The difference will quickly degrade as more rows are fetched in one > query and cache misses and disk head movement start rattling your > disks. The analogy being a man who needs 10 different items from a > supermarket and takes 10 full round trips from home to buy them. Frankly, I didn't even consider fetching database from disk. This slows queries immensely and I wonder if there exist someone who doesn't keep their entire DB in RAM. > I think that the two-tables-one-row lookup will gain the most, > probably even more than 10% I think the gain will raise with the number of tables, because the more tables - the more index lookups are saved. > Adding links does nothing to improve the optimizer, its still free > to choose sucky plans. It is possible that links are faster if used > in the right way, as they cut out the index lookup, but I suspect that > hard-coding link-is-always-faster into the optimiser would also produce > a lot of very bad plans. Methinks that hard-wiring link-is-always-faster into optimizer will still help it very much, because there are few situations where it is not true. > Fixing the optimizer would get a performance gain on a far wider > range of tasks, and is still needed for links. But general fixing of optimizer is total rewritement of it, whereas link fix is almost a fast hack. > So I quess thet if you want links in foreseeable future, your best bet > would be to start coding, and to coordinate with whoever starts to > fix/rewrite > the optimizer (probably Vadim) > Unfortunately I already have a project to work on. There is too little of me for two projects. -- Leon.
Bruce Momjian wrote: > > > > As somone who has developed several commercial dbms systems, > > I would discourage doing a special "link" type. There are > > other ways to gain performance -- de-normalize your tables > > if you are doing mainly reads; carefully check your storage > > layout; and, of course, buy more RAM ;-) > > Good to see you around Bob. This guy does know what he is talking > about. Believe me, I know what I say. Some day I spoke exactly like you, but having seen an impementation of network DBMS, I suddenly realized that SQL days are numbered. The sooner you understand that the better. -- Leon.
Bob Devine wrote: > As somone who has developed several commercial dbms systems, > I would discourage doing a special "link" type. Of course you tried to implement links and failed, didn't you? It such case personally I and maybe others want to hear what can go wrong, in order to benefit from your mistakes and lessons. -- Leon.