Re: [HACKERS] Please Help: PostgreSQL Query Optimizer
Through googling, i found that Normal Disk has external data transfer rate of around 40MBps, ^^ Does this includes, seek and rotational latency ? where as Main Memory has Data transfer rate ranging from 1.6GBps to 2.8GBps. As we can see, the ratio between Disk and Main Memory data transfer rates is around 50. Then, if we multiply all cpu_* paramters by 50, the resulting values will be: random_page_cost = 1; cpu_tuple_cost = 0.5; cpu_index_tuple_cost = 0.05; cpu_operator_cost = 0.0125; Would it be a suitable approach ? We request all of u to give comments/suggestions on this calcualations. Thanking You. On Sun, 11 Dec 2005, Tom Lane wrote: [ trimming cc list to something sane ] Anjan Kumar. A. [EMAIL PROTECTED] writes: In Main Memory DataBase(MMDB) entire database on the disk is loaded on to the main memory during initial startup of the system. There after all the references are made to database on the main memory. When the system is going to shutdown, we will write back the database on the main memory to disk. Here, for the sake of recovery we are writing log records on to the disk during the transaction execution. Don't you get 99.9% of this for free with Postgres' normal behavior? Just increase shared_buffers. Can any one tell me the modifications needs to be incorporated to PostgreSQL, so that it considers only Processing Costs during optimization of the Query. Assuming that a page fetch costs zero is wrong even in an all-in-memory environment. So I don't see any reason you can't maintain the convention that a page fetch costs 1.0 unit, and just adjust the other cost parameters in the light of a different idea about what that actually means. Will it be sufficient, if we change the default values of above paramters in src/include/optimizer/cost.h and src/backend/utils/misc/postgresql.conf.sample as follows: random_page_cost = 4; cpu_tuple_cost = 2; cpu_index_tuple_cost = 0.2; cpu_operator_cost = 0.05; You'd want random_page_cost = 1 since there is presumably no penalty for random access in this context. Also, I think you'd want cpu_operator_cost a lot higher than that (maybe you dropped a decimal place? You scaled the others up by 200 but this one only by 20). It's entirely possible that the ratios of the cpu_xxx_cost values aren't very good and will need work. In the past we've never had occasion to study them very carefully, since they were only marginal contributions anyway. regards, tom lane -- Regards. Anjan Kumar A. MTech2, Comp Sci., www.cse.iitb.ac.in/~anjankumar __ Bradley's Bromide: If computers get too powerful, we can organize them into a committee -- that will do them in. ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] Please Help: PostgreSQL Query Optimizer
Defaulat values of various parameters in PostgreSQL: #random_page_cost = 4 # units are one sequential page fetch cost #cpu_tuple_cost = 0.01 # (same) #cpu_index_tuple_cost = 0.001 # (same) #cpu_operator_cost = 0.0025 # (same) #effective_cache_size = 1000# typically 8KB each Since sequential access is not significantly faster than random access in a MMDB, random_page_cost will be approximately same as sequential page fetch cost. If we make both sequential_page_fetch_cost and random_page_cost to 1, then we need to increase the various cpu_* paramters by multiplying the default values with appropriate Scaling Factor. Now, we need to determine this Scaling Factor. Through googling, i found that Normal Disk has external data transfer rate of around 40MBps, where as Main Memory has Data transfer rate ranging from 1.6GBps to 2.8GBps. As we can see, the ratio between Disk and Main Memory data transfer rates is around 50. Then, if we multiply all cpu_* paramters by 50, the resulting values will be: random_page_cost = 1; cpu_tuple_cost = 0.5; cpu_index_tuple_cost = 0.05; cpu_operator_cost = 0.0125; Would it be a suitable approach ? We request all of u to give comments/suggestions on this calcualations. Thanking You. On Sun, 11 Dec 2005, Tom Lane wrote: [ trimming cc list to something sane ] Anjan Kumar. A. [EMAIL PROTECTED] writes: In Main Memory DataBase(MMDB) entire database on the disk is loaded on to the main memory during initial startup of the system. There after all the references are made to database on the main memory. When the system is going to shutdown, we will write back the database on the main memory to disk. Here, for the sake of recovery we are writing log records on to the disk during the transaction execution. Don't you get 99.9% of this for free with Postgres' normal behavior? Just increase shared_buffers. Can any one tell me the modifications needs to be incorporated to PostgreSQL, so that it considers only Processing Costs during optimization of the Query. Assuming that a page fetch costs zero is wrong even in an all-in-memory environment. So I don't see any reason you can't maintain the convention that a page fetch costs 1.0 unit, and just adjust the other cost parameters in the light of a different idea about what that actually means. Will it be sufficient, if we change the default values of above paramters in src/include/optimizer/cost.h and src/backend/utils/misc/postgresql.conf.sample as follows: random_page_cost = 4; cpu_tuple_cost = 2; cpu_index_tuple_cost = 0.2; cpu_operator_cost = 0.05; You'd want random_page_cost = 1 since there is presumably no penalty for random access in this context. Also, I think you'd want cpu_operator_cost a lot higher than that (maybe you dropped a decimal place? You scaled the others up by 200 but this one only by 20). It's entirely possible that the ratios of the cpu_xxx_cost values aren't very good and will need work. In the past we've never had occasion to study them very carefully, since they were only marginal contributions anyway. regards, tom lane -- Regards. Anjan Kumar A. MTech2, Comp Sci., www.cse.iitb.ac.in/~anjankumar __ A woman physician has made the statement that smoking is neither physically defective nor morally degrading, and that nicotine, even when indulged to in excess, is less harmful than excessive petting. -- Purdue Exponent, Jan 16, 1925 ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] Please Help: PostgreSQL Query Optimizer
On Mon, Dec 12, 2005 at 06:39:42PM +0530, Anjan Kumar. A. wrote: Through googling, i found that Normal Disk has external data transfer rate of around 40MBps, where as Main Memory has Data transfer rate ranging from 1.6GBps to 2.8GBps. I think 40MB/s is a burst speed. You should do some testing to verify. In any case, PostgreSQL doesn't come close to the theoretical maximum disk bandwidth even on a sequential scan. There's been discussion about this on various lists in the past. For a single drive, expect something more in the range of 4-6MB/s (depending on the drive). More important that throughput though, is latency. Because the latency on memory is much closer to 0 (it's not truely 0 due to L1/L2 caching), you can serve concurrent requests a lot faster. -- Jim C. Nasby, Sr. Engineering Consultant [EMAIL PROTECTED] Pervasive Software http://pervasive.comwork: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] Please Help: PostgreSQL Query Optimizer
Anjan, But, in PostgreSQL all costs are scaled relative to a page fetch. If we make both sequential_page_fetch_cost and random_page_cost to 1, then we need to increase the various cpu_* paramters by multiplying the default values with appropriate Scaling Factor. Now, we need to determine this Scaling Factor. I see, so you're saying that because the real cost of a page fetch has decreased, the CPU_* costs should increase proportionally because relative to the real costs of a page fetch they should be higher? That makes a sort of sense. The problem that you're going to run into is that currently we have no particularly reason to believe that the various cpu_* costs are more than very approximately correct as rules of thumb. So I think you'd be a lot better off trying to come up with some means of computing the real cpu costs of each operation, rather than trying to calculate a multiple of numbers which may be wrong in the first place. I know that someone on this list was working on a tool to digest EXPLAIN ANALYZE results and run statistics on them. Can't remember who, though. Also, I'm still curious on how you're handling shared_mem, work_mem and maintenance_mem. You didn't answer last time. -- Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
[HACKERS] Please Help: PostgreSQL Query Optimizer
I'm working on a project, whose implementation deals with PostgreSQL. A brief description of the project is given below. Project Description: In Main Memory DataBase(MMDB) entire database on the disk is loaded on to the main memory during initial startup of the system. There after all the references are made to database on the main memory. When the system is going to shutdown, we will write back the database on the main memory to disk. Here, for the sake of recovery we are writing log records on to the disk during the transaction execution. We want to implement MMDB by modifying PostgreSQL. We implemented our own Main Memory File System to store the primary copy of the database in main memory, and Modified the PostgreSQL to access the data in the Main Memory File System. Now, in our implementation Disk access is completely avoided during normal transaction execution. So, we need to modify the Query Optimizer of PostgreSQL so that it wont consider disk related costs during calculation of Query Costs. Query Optimizer should try to minimize the Processing Cost. The criteria for cost can be taken as the number of tuples that have to read/write from main memory, number of comparisons, etc. Can any one tell me the modifications needs to be incorporated to PostgreSQL, so that it considers only Processing Costs during optimization of the Query. In PostgreSQL, Path costs are measured in units of disk accesses. One sequential page fetch has cost 1. I think, in PostgreSQL following paramters are used in calculating the cost of the Query Path : #random_page_cost = 4 # units are one sequential page fetch cost #cpu_tuple_cost = 0.01 # (same) #cpu_index_tuple_cost = 0.001 # (same) #cpu_operator_cost = 0.0025 # (same) #effective_cache_size = 1000# typically 8KB each In our case we are reading pages from Main Memory File System, but not from Disk. Will it be sufficient, if we change the default values of above paramters in src/include/optimizer/cost.h and src/backend/utils/misc/postgresql.conf.sample as follows: random_page_cost = 4; cpu_tuple_cost = 2; cpu_index_tuple_cost = 0.2; cpu_operator_cost = 0.05; Please help us in this regard. I request all of you to give comments/suggestions on this. Waiting for your kind help. -- Thanks. Anjan Kumar A. MTech2, Comp Sci., www.cse.iitb.ac.in/~anjankumar __ May's Law: The quality of correlation is inversly proportional to the density of control. (The fewer the data points, the smoother the curves.) ---(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] Please Help: PostgreSQL Query Optimizer
Anjan, In our case we are reading pages from Main Memory File System, but not from Disk. Will it be sufficient, if we change the default values of above paramters in src/include/optimizer/cost.h and src/backend/utils/misc/postgresql.conf.sample as follows: random_page_cost = 4; This should be dramatically lowered. It's supposed to represent the ratio of seek-fetches to seq scans on disk. Since there's no disk, it should be a flat 1.0. However, we are aware that there are flaws in our calculations involving random_page_cost, such that the actual number for a system where there is no disk cost would be lower than 1.0. Your research will hopefully help us find these flaws. cpu_tuple_cost = 2; cpu_index_tuple_cost = 0.2; cpu_operator_cost = 0.05; I don't see why you're increasing the various cpu_* costs. CPU costs would be unaffected by the database being in memory. In general, I lower these by a divisor based on the cpu speed; for example, on a dual-opteron system I lower the defaults by /6. However, that's completely unrelated to using an MMDB. So, other than random_page_cost, I don't know of other existing GUCs that would be directly related to using a disk/not using a disk. How are you handling shared memory and work memory? I look forward to hearing more about your test! -- Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [DOCS] [HACKERS] Please Help: PostgreSQL Query Optimizer
Josh Berkus josh@agliodbs.com writes: I don't see why you're increasing the various cpu_* costs. You missed the point Josh --- these numbers are relative to the cost of a page fetch, so if page fetch is measured in microseconds instead of milliseconds, then you *do* want to bump the CPU costs up. regards, tom lane ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] Please Help: PostgreSQL Query Optimizer
Since sequential access is not significantly faster than random access in a MMDB, random_page_cost will be approximately same as sequential page fetch cost. As every thing is present in Main Memory, we need to give approximately same cost to read/write to Main Memory and CPU Related operations. But, in PostgreSQL all costs are scaled relative to a page fetch. If we make both sequential_page_fetch_cost and random_page_cost to 1, then we need to increase the various cpu_* paramters by multiplying the default values with appropriate Scaling Factor. Now, we need to determine this Scaling Factor. Still, i want to confirm whether this approach is the correct one. On Sun, 11 Dec 2005, Josh Berkus wrote: Anjan, In our case we are reading pages from Main Memory File System, but not from Disk. Will it be sufficient, if we change the default values of above paramters in src/include/optimizer/cost.h and src/backend/utils/misc/postgresql.conf.sample as follows: random_page_cost = 4; This should be dramatically lowered. It's supposed to represent the ratio of seek-fetches to seq scans on disk. Since there's no disk, it should be a flat 1.0. However, we are aware that there are flaws in our calculations involving random_page_cost, such that the actual number for a system where there is no disk cost would be lower than 1.0. Your research will hopefully help us find these flaws. cpu_tuple_cost = 2; cpu_index_tuple_cost = 0.2; cpu_operator_cost = 0.05; I don't see why you're increasing the various cpu_* costs. CPU costs would be unaffected by the database being in memory. In general, I lower these by a divisor based on the cpu speed; for example, on a dual-opteron system I lower the defaults by /6. However, that's completely unrelated to using an MMDB. So, other than random_page_cost, I don't know of other existing GUCs that would be directly related to using a disk/not using a disk. How are you handling shared memory and work memory? I look forward to hearing more about your test! -- Regards. Anjan Kumar A. MTech2, Comp Sci., www.cse.iitb.ac.in/~anjankumar __ Do not handicap your children by making their lives easy. -- Robert Heinlein ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] Please Help: PostgreSQL Query Optimizer
Hi,
I'm very new to this list -- I've been using and advocating PostgreSQL for
no less than 4 or 5 years now, and have participated in some of the other
mailing lists, but never on this one.
My question is (short version): how would one go about adding a new
(built-in) function to PostgreSQL?
Long-ish version:
I know the answer in theory -- one goes through the source code, find
out how it all works, and modify/add the code to add or fix whatever
feature we want.
I guess my point in here would be rather a feature request -- except
that I'd find it pretty exciting to implement it myself, and then propose
the new feature by volunteering the implementation that I already wrote
(seems like the spirit of open-source communities, right?) -- then of
course, it would be subject to consensus, whether or not the feature
makes sense and the implementation is good enough.
I'm interested in adding additional hash functions -- PG supports, as part
of the built-in SQL functions, MD5 hashing. So, for instance, I can simply
type, at a psql console, the following:
select md5('abc');
My feature request (which again, I'd like to implement it myself) would
be the ability to do:
select sha1('xyz'), sha256('etc');
(At least these two -- maybe for completeness it would be good to have
sha224, 384, and 512, but I guess SHA1 and SHA-256 would be a very good
and sound starting point)
So, can you offer some advice or pointers on how to go about that?
I started by doing a search for the string md5 through all the source
code -- the problem is, md5 shows up in many many many places (it is
part of the authentication protocol, among other things), so I got a
little bit lost searching through it all.
I wonder if you have some documents specifically aimed at providing
advice and documentation for prospective developers (or for people
that want to tweak the source code to fix/tuneup or add functionality),
I guess that would be great for me in this case.
Thanks!
Carlos
--
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Adding funtions to postgresql (Not - )e: [HACKERS] Please Help: PostgreSQL Query Optimizer
Ühel kenal päeval, P, 2005-12-11 kell 17:55, kirjutas Carlos Moreno:
Hi,
I'm very new to this list -- I've been using and advocating PostgreSQL for
no less than 4 or 5 years now, and have participated in some of the other
mailing lists, but never on this one.
My question is (short version): how would one go about adding a new
(built-in) function to PostgreSQL?
Ask your question as a separate post, not as an answer t another
thread :)
I'm interested in adding additional hash functions -- PG supports, as part
of the built-in SQL functions, MD5 hashing. So, for instance, I can simply
type, at a psql console, the following:
select md5('abc');
My feature request (which again, I'd like to implement it myself) would
be the ability to do:
select sha1('xyz'), sha256('etc');
(At least these two -- maybe for completeness it would be good to have
sha224, 384, and 512, but I guess SHA1 and SHA-256 would be a very good
and sound starting point)
Take a look at the pgcrypto contrib module (in src/contrib/pgcrypto )
Not only does it show how to add functions, but it also provides many of
the ones you need.
Hannu
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Re: [HACKERS] Please Help: PostgreSQL Query Optimizer
Actually, there is probably comparatively little to gain from making it
a builtin. And SHA1 is already there in the pgcrypto contrib module.
Presumably if we wanted a builtin we would start from that code base.
cheers
andrew
Carlos Moreno wrote:
Hi,
I'm very new to this list -- I've been using and advocating PostgreSQL
for
no less than 4 or 5 years now, and have participated in some of the other
mailing lists, but never on this one.
My question is (short version): how would one go about adding a new
(built-in) function to PostgreSQL?
Long-ish version:
I know the answer in theory -- one goes through the source code, find
out how it all works, and modify/add the code to add or fix whatever
feature we want.
I guess my point in here would be rather a feature request -- except
that I'd find it pretty exciting to implement it myself, and then propose
the new feature by volunteering the implementation that I already wrote
(seems like the spirit of open-source communities, right?) -- then of
course, it would be subject to consensus, whether or not the feature
makes sense and the implementation is good enough.
I'm interested in adding additional hash functions -- PG supports, as
part
of the built-in SQL functions, MD5 hashing. So, for instance, I can
simply
type, at a psql console, the following:
select md5('abc');
My feature request (which again, I'd like to implement it myself) would
be the ability to do:
select sha1('xyz'), sha256('etc');
(At least these two -- maybe for completeness it would be good to have
sha224, 384, and 512, but I guess SHA1 and SHA-256 would be a very good
and sound starting point)
So, can you offer some advice or pointers on how to go about that?
I started by doing a search for the string md5 through all the source
code -- the problem is, md5 shows up in many many many places (it is
part of the authentication protocol, among other things), so I got a
little bit lost searching through it all.
I wonder if you have some documents specifically aimed at providing
advice and documentation for prospective developers (or for people
that want to tweak the source code to fix/tuneup or add functionality),
I guess that would be great for me in this case.
---(end of broadcast)---
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Re: [HACKERS] Please Help: PostgreSQL Query Optimizer
Am Sonntag, den 11.12.2005, 17:55 -0500 schrieb Carlos Moreno:
...
I'm interested in adding additional hash functions -- PG supports, as part
of the built-in SQL functions, MD5 hashing. So, for instance, I can simply
type, at a psql console, the following:
select md5('abc');
My feature request (which again, I'd like to implement it myself) would
be the ability to do:
select sha1('xyz'), sha256('etc');
(At least these two -- maybe for completeness it would be good to have
sha224, 384, and 512, but I guess SHA1 and SHA-256 would be a very good
and sound starting point)
So, can you offer some advice or pointers on how to go about that?
You might want to check out contrib/pgcrypto
more often then not, if you want something, its
already done ;)
Not sure if this will ever be included in the core,
since not many people need these advanced hash functions.
HTH
Tino Wildenhain
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