Re: [PERFORM] Indexes with descending date columns
Hi, Bruce, Bruce Momjian wrote: Ahh. There's a hack to do that by defining a new opclass that reverses and , and then doing ORDER BY project_id, id, date USING new_opclass. I think there's a TODO about this, but I'm not sure... Yes, and updated: * Allow the creation of indexes with mixed ascending/descending specifiers This is possible now by creating an operator class with reversed sort operators. One complexity is that NULLs would then appear at the start of the result set, and this might affect certain sort types, like merge join. I think it would be better to allow index zig-zag scans for multi-column index.[1] So it traverses in a given order on the higher order column, and the sub trees for each specific high order value is traversed in reversed order. From my knowledge at least of BTrees, and given correct commutator definitions, this should be not so complicated to implement.[2] This would allow the query planner to use the same index for arbitrary ASC/DESC combinations of the given columns. Just a thought, Markus [1] It may make sense to implement the mixed specifiers on indices as well, to allow CLUSTERing on mixed search order. [2] But I admit that I currently don't have enough knowledge in PostgreSQL index scan internals to know whether it really is easy to implement. -- Markus Schaber | Logical TrackingTracing International AG Dipl. Inf. | Software Development GIS Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Indexes with descending date columns
Jim C. Nasby wrote: On Wed, Mar 29, 2006 at 12:52:31PM +0200, Theo Kramer wrote: On Fri, 2006-03-24 at 12:21, Jim C. Nasby wrote: On Thu, Mar 23, 2006 at 01:09:49PM +0200, Theo Kramer wrote: ii If no to i, is it feasible to extend PostgreSQL to allow traversing an index in column descending and column ascending order - assuming an order by on more than one column with column order not in the same direction and indexes existing? ... if that makes sense. Yes. stats=# explain select * from email_contrib order by project_id desc, id desc, date desc limit 10; QUERY PLAN Limit (cost=0.00..31.76 rows=10 width=24) - Index Scan Backward using email_contrib_pkey on email_contrib (cost=0.00..427716532.18 rows=134656656 width=24) (2 rows) Not quite what I mean - redo the above as follows and then see what explain returns explain select * from email_contrib order by project_id, id, date desc limit 10; Ahh. There's a hack to do that by defining a new opclass that reverses and , and then doing ORDER BY project_id, id, date USING new_opclass. I think there's a TODO about this, but I'm not sure... Yes, and updated: * Allow the creation of indexes with mixed ascending/descending specifiers This is possible now by creating an operator class with reversed sort operators. One complexity is that NULLs would then appear at the start of the result set, and this might affect certain sort types, like merge join. -- Bruce Momjian http://candle.pha.pa.us + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PERFORM] Indexes with descending date columns
On Wed, Mar 29, 2006 at 12:52:31PM +0200, Theo Kramer wrote: On Fri, 2006-03-24 at 12:21, Jim C. Nasby wrote: On Thu, Mar 23, 2006 at 01:09:49PM +0200, Theo Kramer wrote: ii If no to i, is it feasible to extend PostgreSQL to allow traversing an index in column descending and column ascending order - assuming an order by on more than one column with column order not in the same direction and indexes existing? ... if that makes sense. Yes. stats=# explain select * from email_contrib order by project_id desc, id desc, date desc limit 10; QUERY PLAN Limit (cost=0.00..31.76 rows=10 width=24) - Index Scan Backward using email_contrib_pkey on email_contrib (cost=0.00..427716532.18 rows=134656656 width=24) (2 rows) Not quite what I mean - redo the above as follows and then see what explain returns explain select * from email_contrib order by project_id, id, date desc limit 10; Ahh. There's a hack to do that by defining a new opclass that reverses and , and then doing ORDER BY project_id, id, date USING new_opclass. I think there's a TODO about this, but I'm not sure... -- 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 6: explain analyze is your friend
Re: [PERFORM] Indexes with descending date columns
On Fri, 2006-03-24 at 12:21, Jim C. Nasby wrote: On Thu, Mar 23, 2006 at 01:09:49PM +0200, Theo Kramer wrote: ii If no to i, is it feasible to extend PostgreSQL to allow traversing an index in column descending and column ascending order - assuming an order by on more than one column with column order not in the same direction and indexes existing? ... if that makes sense. Yes. stats=# explain select * from email_contrib order by project_id desc, id desc, date desc limit 10; QUERY PLAN Limit (cost=0.00..31.76 rows=10 width=24) - Index Scan Backward using email_contrib_pkey on email_contrib (cost=0.00..427716532.18 rows=134656656 width=24) (2 rows) Not quite what I mean - redo the above as follows and then see what explain returns explain select * from email_contrib order by project_id, id, date desc limit 10; -- Regards Theo ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [PERFORM] Indexes with descending date columns
On Thu, Mar 23, 2006 at 01:09:49PM +0200, Theo Kramer wrote: ii If no to i, is it feasible to extend PostgreSQL to allow traversing an index in column descending and column ascending order - assuming an order by on more than one column with column order not in the same direction and indexes existing? ... if that makes sense. Yes. stats=# explain select * from email_contrib order by project_id desc, id desc, date desc limit 10; QUERY PLAN Limit (cost=0.00..31.76 rows=10 width=24) - Index Scan Backward using email_contrib_pkey on email_contrib (cost=0.00..427716532.18 rows=134656656 width=24) (2 rows) -- 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 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: [PERFORM] Indexes with descending date columns
On Fri, 2006-03-17 at 08:25, [EMAIL PROTECTED] wrote: I have a performance problem when traversing a table in index order with multiple columns including a date column in date reverse order. Below follows a simplified description of the table, the index and the associated query \d prcdedit prcdedit_prcd | character(20) | prcdedit_date | timestamp without time zone | Indexes: prcdedit_idx btree (prcdedit_prcd, prcdedit_date) Depending on how you use the table, there are three possible solutions. First, if it makes sense in the domain, using an ORDER BY where _both_ columns are used descending will make PG search the index in reverse and will be just as fast as when both as searched by the default ascending. Second possibility: Create a dummy column whose value depends on the negative of prcdedit_date, e.g., -extract(epoch from prcdedit_date), keep the dummy column in sync with the original column using triggers, and rewrite your queries to use ORDER BY prcdedit_prod, dummy_column. Third: Create an index on a function which sorts in the order you want, and then always sort using the function index (you could use the -extract(epoch...) gimmick for that, among other possibilities.) HTH. All good input - thanks, however, before I start messing with my stuff which I know will be complex - some questions to any of the developers on the list. i Is it feasible to extend index creation to support descending columns? ... this is supported on other commercial and non commercial databases, but I do not know if this is a SQL standard. ii If no to i, is it feasible to extend PostgreSQL to allow traversing an index in column descending and column ascending order - assuming an order by on more than one column with column order not in the same direction and indexes existing? ... if that makes sense. -- Regards Theo ---(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: [PERFORM] Indexes with descending date columns
Theo Kramer wrote: All good input - thanks, however, before I start messing with my stuff which I know will be complex - some questions to any of the developers on the list. i Is it feasible to extend index creation to support descending columns? ... this is supported on other commercial and non commercial databases, but I do not know if this is a SQL standard. This can be done. You need to create an operator class which specifies the reverse sort order (i.e. reverse the operators), and then use it in the new index. -- Alvaro Herrerahttp://www.CommandPrompt.com/ The PostgreSQL Company - Command Prompt, Inc. ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Indexes with descending date columns
On Thu, 2006-03-23 at 16:16, Alvaro Herrera wrote: Theo Kramer wrote: All good input - thanks, however, before I start messing with my stuff which I know will be complex - some questions to any of the developers on the list. i Is it feasible to extend index creation to support descending columns? ... this is supported on other commercial and non commercial databases, but I do not know if this is a SQL standard. This can be done. You need to create an operator class which specifies the reverse sort order (i.e. reverse the operators), and then use it in the new index. Hmmm, would that then result in the following syntax being valid? create index my_idx on my_table (c1, c2 desc, c3, c4 desc) ; where my_table is defined as create table my_table ( c1 text, c2 timestamp, c3 integer, c4 integer ); If so, I would appreciate any pointers on where to start on this - already fumbling my way through Interfacing Extensions To Indexes in the manual... Regards Theo -- Regards Theo ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Indexes with descending date columns
Theo Kramer [EMAIL PROTECTED] writes: If so, I would appreciate any pointers on where to start on this - already fumbling my way through Interfacing Extensions To Indexes in the manual... Search the PG list archives for discussions of reverse-sort opclasses. It's really pretty trivial, once you've created a negated btree comparison function for the datatype. This is the sort of thing that we are almost but not quite ready to put into the standard distribution. The issues that are bugging me have to do with whether NULLs sort low or high --- right now, if you make a reverse-sort opclass, it will effectively sort NULLs low instead of high, and that has some unpleasant consequences because the rest of the system isn't prepared for variance on the point (in particular I'm afraid this could break mergejoins). I'd like to see us make NULLs low vs NULLs high be a defined property of opclasses, and deal with the fallout from that, and then we could put reverse-sort opclasses for all the standard datatypes into the regular distribution. regards, tom lane ---(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
[PERFORM] Indexes with descending date columns
Hi I have a performance problem when traversing a table in index order with multiple columns including a date column in date reverse order. Below follows a simplified description of the table, the index and the associated query \d prcdedit prcdedit_prcd | character(20) | prcdedit_date | timestamp without time zone | Indexes: prcdedit_idx btree (prcdedit_prcd, prcdedit_date) When invoking a query such as select oid, prcdedit_prcd, prcdedit_date, 'dd/mm/ hh24:mi:ss') as mydate where prcdedit_prcd 'somevalue' order by prcdedit_prcd, prcdedit_date desc; the peformance is dismal. However removing the 'desc' qualifier as follows the query flys select oid, prcdedit_prcd, prcdedit_date, 'dd/mm/ hh24:mi:ss') as mydate where prcdedit_prcd 'somevalue' order by prcdedit_prcd, prcdedit_date; PostgreSQL Version = 8.1.2 Row count on the table is 30 Explain is as follows for desc Limit (cost=81486.35..81486.41 rows=25 width=230) (actual time=116619.652..116619.861 rows=25 loops=1) - Sort (cost=81486.35..82411.34 rows=369997 width=230) (actual time=116619.646..116619.729 rows=25 loops=1) Sort Key: prcdedit_prcd, prcdedit_date, oid - Bitmap Heap Scan on prcdedit (cost=4645.99..23454.94 rows=369997 width=230) (actual time=376.952..11798.834 rows=369630 loops=1) Recheck Cond: (prcdedit_prcd '063266 '::bpchar) - Bitmap Index Scan on prcdedit_idx (cost=0.00..4645.99 rows=369997 width=0) (actual time=366.048..366.048 rows=369630 loops=1) Index Cond: (prcdedit_prcd '063266 '::bpchar) Total runtime: 116950.175 ms and as follows when I remove the 'desc' Limit (cost=0.00..2.34 rows=25 width=230) (actual time=0.082..0.535 rows=25 loops=1) - Index Scan using prcdedit_idx on prcdedit (cost=0.00..34664.63 rows=369997 width=230) (actual time=0.075..0.405 rows=25 loops=1) Index Cond: (prcdedit_prcd '063266 '::bpchar) Total runtime: 0.664 ms Any assistance/advice much appreciated. -- Regards Theo ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PERFORM] Indexes with descending date columns
I have a performance problem when traversing a table in index order with multiple columns including a date column in date reverse order. Below follows a simplified description of the table, the index and the associated query \d prcdedit prcdedit_prcd | character(20) | prcdedit_date | timestamp without time zone | Indexes: prcdedit_idx btree (prcdedit_prcd, prcdedit_date) Depending on how you use the table, there are three possible solutions. First, if it makes sense in the domain, using an ORDER BY where _both_ columns are used descending will make PG search the index in reverse and will be just as fast as when both as searched by the default ascending. Second possibility: Create a dummy column whose value depends on the negative of prcdedit_date, e.g., -extract(epoch from prcdedit_date), keep the dummy column in sync with the original column using triggers, and rewrite your queries to use ORDER BY prcdedit_prod, dummy_column. Third: Create an index on a function which sorts in the order you want, and then always sort using the function index (you could use the -extract(epoch...) gimmick for that, among other possibilities.) HTH. ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
