Thread: SELECT ... LIMIT (trial implementation)
Here we go, this is up to now only for discussion, do not apply to CVS! Those involved into the LIMIT discussion please comment. Here is what I had in mind for the SELECT ... LIMIT. It adds SELECT ... [LIMIT count [, offset]] to the parser and arranges that these values are passed down to the executor. It is a clean implementation of LIMIT (regression tested) and the open items on it are to enable parameters and handle it in SQL functions and SPI stuff (currently ignored in both). Optimizing the executor would require the other sort node stuff discussion first to come to a conclusion. For now it skips final result rows - but that's already one step forward since it reduces the rows sent to the frontend to exactly that what LIMIT requested. I've seen the queryLimit by SET variable stuff and that really can break rewrite rules, triggers or functions. This is because the query limit will be inherited by any query (inserts, updates, deletes too) done by them. Have a rule for constraint deletes of referencing tuples CREATE RULE del_table1 AS ON DELETE TO table1 DO DELETE FROM table2 WHERE ref = OLD.key; If the user now sets the query limit to 1 via SET and deletes a row from table1, only the first found record in table2 will be constraint deleted, not all of them. This is a feature where users can get around rules that ensure data integrity. Jan -- #======================================================================# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #======================================== jwieck@debis.com (Jan Wieck) # begin 644 opt_limit.diff.gz M'XL(`$]=)S8"`^4\:W?B1K*?R:_H82=98&1;$F^<>`^+F1DV&!S`N<DGCBP: M6SM"(I*PQSOQ?[]5_9!:(`$S<1Y[+V<&I.[JZJ[NZGIUM1?.<DE.[("$@7WJ M!\[=V:UE?Z#>XLSV5RO+6X3RX=1&F#W57U4JE6/0%-XAG;$3$:Q#`Z]6JG MVB)&N]WZZN3DY$`?J;9FI][HU&J\;27]88.IMC6H9J^(&E_K!)Y.OB+D;XYG MNYL%)<5-Y+CAV<K^&)W>%S-JUGX066YV7?@4VI9]3['VC5(;1H'CW?$V\.^L MPKI5/U!**H7"-4/>_TCM3>0'[ZFUOJ(K/W@B8;19+C.I,G1#,_1&0I>AFU#0 MDI05'"\J%`I+:D6;@)YCR0\;&CQ=TM"&/G^1SZR&]];SO8A^C(C-?\^!E@*4 MA8#'=5:VO_&B<TY)(9N4`M`26IX3/1&8#?M#F#URHZH91DL9N5'7C&HM'OGS MGD[>L$YZ`9!%B85##2/B^3#;R\!?D>B>DCOG@7J$#9<\6.Z&\D8YN,[P9T57 M(8U*WT@R-:)K)'3^0_UE29:5RVQ"Y.MI]+2FA<)W9#9G<Y2N9.-B$-^1P6A6 M&P\N,P!<ZD&]Z`?6JU;.`&(D`%CI$E9R51;+L`/FA-[&=0'N;7<X[6<`W#X! 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jwieck@debis.com (Jan Wieck) writes: > I've seen the queryLimit by SET variable stuff and that > really can break rewrite rules, triggers or functions. This > is because the query limit will be inherited by any query > (inserts, updates, deletes too) done by them. > [ example snipped ] > This is a feature where users can get around rules that > ensure data integrity. Ouch. I think this point is a *fatal* objection to implementing query limit as a SET variable. That might be a quick-and-dirty way of getting some functionality going, but we can't let it loose on the world like that. regards, tom lane
> jwieck@debis.com (Jan Wieck) writes: > > I've seen the queryLimit by SET variable stuff and that > > really can break rewrite rules, triggers or functions. This > > is because the query limit will be inherited by any query > > (inserts, updates, deletes too) done by them. > > [ example snipped ] > > This is a feature where users can get around rules that > > ensure data integrity. > > Ouch. I think this point is a *fatal* objection to implementing > query limit as a SET variable. That might be a quick-and-dirty way > of getting some functionality going, but we can't let it loose on the > world like that. OK, I assume you are saying that you like LIMIT/OFFSET in the query, but not as a SET command that could be unreliable. Jan has already coded a much more reliable, user-friently way, by putting the LIMIT/OFFSET in the query, and I think that is the way to go too. -- 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, Pennsylvania 19026
> Here we go, > > this is up to now only for discussion, do not apply to CVS! > > Those involved into the LIMIT discussion please comment. > > Here is what I had in mind for the SELECT ... LIMIT. It adds > > SELECT ... [LIMIT count [, offset]] > > to the parser and arranges that these values are passed down > to the executor. My only suggestion is that I don't like syntax where you have value 'a,b', and a and b have different meanings. I would prefer: SELECT ... [LIMIT count [OFFSET offset]] This makes things much clearer for people reading the query. What if someone wants the rows from 500 to the end. Should we allow the syntax to be: SELECT ... [LIMIT count] [OFFSET offset] LIMIT and OFFSET are independent. > It is a clean implementation of LIMIT (regression tested) and > the open items on it are to enable parameters and handle it > in SQL functions and SPI stuff (currently ignored in both). > Optimizing the executor would require the other sort node > stuff discussion first to come to a conclusion. For now it > skips final result rows - but that's already one step forward > since it reduces the rows sent to the frontend to exactly > that what LIMIT requested. > > I've seen the queryLimit by SET variable stuff and that > really can break rewrite rules, triggers or functions. This > is because the query limit will be inherited by any query > (inserts, updates, deletes too) done by them. Have a rule for > constraint deletes of referencing tuples > > CREATE RULE del_table1 AS ON DELETE TO table1 DO > DELETE FROM table2 WHERE ref = OLD.key; > > If the user now sets the query limit to 1 via SET and deletes > a row from table1, only the first found record in table2 will > be constraint deleted, not all of them. > > This is a feature where users can get around rules that > ensure data integrity. OK, I am all for removal of SET QUERY_LIMIT, especially if we think we can get something better in a post-6.4 release. I assume the current strategy for impelemting LIMIT..OFFSET is: For single-table queries, if the index matches the ORDER BY, use the index to do the LIMIT..OFFSET. Large offset value require a sequential scan of the index until it reaches the OFFSET. For joins, if an index matches the ORDER BY, and the indexed table is on the outside of a join loop, use the index to force the query to execute in ORDER BY order, and reduce the number of values in the query. It would be nifty if we could peek into the index and change LIMIT to an actual range of value that would automatically match an index, then you have to force the optimizer to use the index, i.e. SELECT * FROM tab LIMIT 100 becomes: SELECT * FROM tab WHERE x < 732 but that is very strange to do, and I would prefer not to approach it that way. Seems like Jan has already done it better than that. -- 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, Pennsylvania 19026
Hi all, currently CVS is not working. It gives the following error message: marliesle:/usr/local/pgsql# cvs -z3 update -d Fatal error, aborting. : no such user -Egon
Bruce Momjian <maillist@candle.pha.pa.us> writes: > OK, I assume you are saying that you like LIMIT/OFFSET in the query, but > not as a SET command that could be unreliable. Precisely. Adding LIMIT/OFFSET options to SELECT sounds like a good idea for all the reasons previously given. But the SET QUERYLIMIT command is positively dangerous --- I think we should take it out. regards, tom lane
Bruce Momjian <maillist@candle.pha.pa.us> writes: > What if someone wants the rows from 500 to the end. Should we allow > the syntax to be: > SELECT ... [LIMIT count] [OFFSET offset] > LIMIT and OFFSET are independent. I like that syntax the best, but remember we are not inventing in a green field here. Isn't this a feature that already exists in other DBMs? We should probably copy their syntax, unless it's truly spectacularly awful... regards, tom lane
On Sun, 18 Oct 1998, Tom Lane wrote: > Date: Sun, 18 Oct 1998 12:04:49 -0400 > From: Tom Lane <tgl@sss.pgh.pa.us> > To: Bruce Momjian <maillist@candle.pha.pa.us> > Cc: pgsql-hackers@postgreSQL.org > Subject: Re: [HACKERS] SELECT ... LIMIT (trial implementation) > > Bruce Momjian <maillist@candle.pha.pa.us> writes: > > What if someone wants the rows from 500 to the end. Should we allow > > the syntax to be: > > SELECT ... [LIMIT count] [OFFSET offset] > > LIMIT and OFFSET are independent. > > I like that syntax the best, but remember we are not inventing in > a green field here. Isn't this a feature that already exists in > other DBMs? We should probably copy their syntax, unless it's > truly spectacularly awful... > > regards, tom lane > Mysql uses LIMIT [offset,] rows From documentation: LIMIT takes one or two numeric arguments. A single argument represents the maximum number of rows to return in a result. If two arguments are given the first argument is the offset to the first row to return, while the second is the maximum number of rows to return in the result. What would be nice if somehow total number of rows could be returned. This is often needed for altavista-like application. Of course, I can do select count(*) from sometable ... LIMIT offset, rows and then select ... from sometable ... LIMIT offset, rows but this seems not elegant solution. Regards, Oleg _____________________________________________________________ Oleg Bartunov, sci.researcher, hostmaster of AstroNet, Sternberg Astronomical Institute, Moscow University (Russia) Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/ phone: +007(095)939-16-83, +007(095)939-23-83
Jan, I tested your patch on my Linux box and it works ok, except aggregates functions doesn't work properly, for example count(*) always produces 0 kdo=> select count(*) from work_flats limit 10,1000; count ----- (0 rows) while kdo=> select rooms from work_flats limit 10,1000; rooms ----- 3 3 3 3 3 3 3 3 3 3 (10 rows) Regards, Oleg _____________________________________________________________ Oleg Bartunov, sci.researcher, hostmaster of AstroNet, Sternberg Astronomical Institute, Moscow University (Russia) Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/ phone: +007(095)939-16-83, +007(095)939-23-83
Oleg Bartunov wrote: > > Jan, > > I tested your patch on my Linux box and it works ok, except > aggregates functions doesn't work properly, for example > count(*) always produces 0 They work absolutely properly :-) > > kdo=> select count(*) from work_flats limit 10,1000; > count > ----- > (0 rows) As I wrote, the executor skips final result rows. In the obove query, there is only one result row (the one returned by the aggregate function). You asked the executor to skip it and it did. We cannot limit selections in deeper levels than the top one. This would give unpredictable results in joins. Jan -- #======================================================================# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #======================================== jwieck@debis.com (Jan Wieck) #
Oleg Bartunov wrote: > On Sun, 18 Oct 1998, Tom Lane wrote: > > > Bruce Momjian <maillist@candle.pha.pa.us> writes: > > > What if someone wants the rows from 500 to the end. Should we allow > > > the syntax to be: > > > SELECT ... [LIMIT count] [OFFSET offset] > > > LIMIT and OFFSET are independent. > > > > I like that syntax the best, but remember we are not inventing in > > a green field here. Isn't this a feature that already exists in > > other DBMs? We should probably copy their syntax, unless it's > > truly spectacularly awful... > > > > regards, tom lane > > > > Mysql uses LIMIT [offset,] rows > >From documentation: > > LIMIT takes one or two numeric arguments. A single argument > represents the maximum number of rows to return in a result. If two > arguments are given the first argument is the offset to the first row to > return, while the second is the maximum number of rows to return in the > result. Simple change, just flip them in gram.y. And for the 500 to end: SELECT ... LIMIT 500, 0 (after flipped) The 0 has the same meaning as ALL. And that could also be added to the parser easily so one can say SELECT ... LIMIT 500, ALL too. > > What would be nice if somehow total number of rows could be returned. > This is often needed for altavista-like application. > Of course, I can do > select count(*) from sometable ... LIMIT offset, rows > and then > select ... from sometable ... LIMIT offset, rows > but this seems not elegant solution. Absolutely makes no sense for me. As said in the other posting, aggregates do the counting scan in a deeper level and thus cannot get limited. So if you invoke an aggregate, the whole scan is always done. Jan -- #======================================================================# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #======================================== jwieck@debis.com (Jan Wieck) #
On Sun, 18 Oct 1998, Tom Lane wrote: > Bruce Momjian <maillist@candle.pha.pa.us> writes: > > What if someone wants the rows from 500 to the end. Should we allow > > the syntax to be: > > SELECT ... [LIMIT count] [OFFSET offset] > > LIMIT and OFFSET are independent. > > I like that syntax the best, but remember we are not inventing in > a green field here. Isn't this a feature that already exists in > other DBMs? We should probably copy their syntax, unless it's > truly spectacularly awful... > > regards, tom lane None that I have used (VFP, M$ SQL Server) that had 'LIMIT', had 'OFFSET'. So it would seem that the very idea of OFFSET is to break with what others are doing. I too like the above syntax. Why mimic, when you can do better? Go for it! Just my vote, have a great day Terry Mackintosh <terry@terrym.com> http://www.terrym.com sysadmin/owner Please! No MIME encoded or HTML mail, unless needed. Proudly powered by R H Linux 4.2, Apache 1.3, PHP 3, PostgreSQL 6.3 ------------------------------------------------------------------- Success Is A Choice ... book by Rick Patino, get it, read it!
> > On Sun, 18 Oct 1998, Tom Lane wrote: > > > Bruce Momjian <maillist@candle.pha.pa.us> writes: > > > What if someone wants the rows from 500 to the end. Should we allow > > > the syntax to be: > > > SELECT ... [LIMIT count] [OFFSET offset] > > > LIMIT and OFFSET are independent. > > > > I like that syntax the best, but remember we are not inventing in > > a green field here. Isn't this a feature that already exists in > > other DBMs? We should probably copy their syntax, unless it's > > truly spectacularly awful... > > > > regards, tom lane > > None that I have used (VFP, M$ SQL Server) that had 'LIMIT', had 'OFFSET'. > So it would seem that the very idea of OFFSET is to break with what others > are doing. > > I too like the above syntax. > Why mimic, when you can do better? Go for it! > We have a powerful parser. So we can provide ... [ LIMIT { rows | ALL } ] [ OFFSET skip ] or ... [ LIMIT [ skip , ] { rows | ALL } ] at the same time. Jan -- #======================================================================# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #======================================== jwieck@debis.com (Jan Wieck) #
Hi folks, Fatal error, aborting. : no such user -Egon
I said: > It is a clean implementation of LIMIT (regression tested) and > the open items on it are to enable parameters and handle it > in SQL functions and SPI stuff (currently ignored in both). > Optimizing the executor would require the other sort node > stuff discussion first to come to a conclusion. For now it > skips final result rows - but that's already one step forward > since it reduces the rows sent to the frontend to exactly > that what LIMIT requested. Parameters - done SPI stuff - done SQL functions - no LIMIT (cannot work) For SPI calls, a LIMIT clause in the query will take precedence over the tcount argument to SPI_exec()/SPI_execp(). So SPI functions stay 100% backward compatible, but LIMIT is also available for C and PL functions. Unfortunately code is frozen. And since this is feature, it is past 6.4. Or can we get it out of the refrigerator for a moment, Marc? Jan -- #======================================================================# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #======================================== jwieck@debis.com (Jan Wieck) #
> We have a powerful parser. So we can provide > [...] This version now accepts all of the following ... [ LIMIT rows ] [ OFFSET skip ] ... [ OFFSET skip ] [ LIMIT rows ] ... [ LIMIT [ skip , ] rows ] rows can be a positive integer constant greater that 0, a $n parameter (in SPI_prepare()) or the keyword ALL. 0 isn't accepted as constant to force ALL in that case making clear that this is wanted. In the parameter version the integer value 0 still is used to mean ALL. skip can be a positive integer constant greater or equal to 0 or a $n parameter for SPI_prepare. If any of these syntaxes is used in SPI_prepare()'d plans, the given tcount argument for SPI_execp() is ignored and the plan or parameter values are used. Anyone happy now? 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