Question

Postgres SQL: Best to find date range conflict

Asked by: Thirt

I use the sql code below to find if an existing record exist for the same resource in the same date/time range.  So the resource 'Conference Room B' can not be double booked.

There is an existing record on 10/27/2009 from 8:00 - 9:00 am. The below sql is finding a match so as a work around, the user needs to specify 7:30 am - 7:45 to book the room before the existing reservation. So I think this is a more general SQL question. That is, what is the best way to test if an existing resource block of time conflicts with an existing record?

Idealy, I would like the user to specify the true time they need the room, from 7:30 am - 8:00 am.

select dat_indexno, dat_resource from plus.tbl_waldo where dat_resource = 'Conference Room B' and ('10/27/2009 7:30:00 AM' <= dat_to) AND ('10/27/2009 8:00:00 AM' >= dat_from)"

                                  
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Asked On
2009-10-26 at 15:44:59ID24845562
Topics

PostgreSQL Database

,

SQL Query Syntax

Participating Experts
2
Points
500
Comments
6

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Answers

 

by: DrCabbagePosted on 2009-10-26 at 16:54:49ID: 25668177

There's an OVERLAPS operator that does this for you: http://www.postgresql.org/docs/8.4/interactive/functions-datetime.html

The example below assumes that your current datestyle settings recognise those date/time values appropriately)

select dat_indexno, dat_resource from plus.tbl_waldo 
  where dat_resource = 'Conference Room B' 
    and (dat_from,dat_to) overlaps ('10/27/2009 7:30:00 AM', '10/27/2009 8:00:00 AM')

                                              
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by: ThirtPosted on 2009-10-27 at 07:28:05ID: 25672770

I'm getting the following error:


ERROR: function pg_catalog.overlaps(timestamp with time zone, timestamp with time zone, "unknown", "unknown") is not unique
SQL state: 42725
Hint: Could not choose a best candidate function. You may need to add explicit type casts.

 

by: ThirtPosted on 2009-10-27 at 07:32:50ID: 25672819

I kinda inherited this Postgres table and not sure why timezone with be used? We have only one application from one location inserting records. Do I or should I need to change this?

CREATE TABLE plus.tbl_waldo
(
  dat_indexno integer NOT NULL DEFAULT nextval('plus.waldo_id1'::text),
  dat_empname character varying(50) NOT NULL,
  dat_type character varying(1) NOT NULL,
  dat_resource character varying(30),
  dat_from timestamp with time zone NOT NULL,
  dat_to timestamp with time zone NOT NULL,
  dat_note character varying(100),
  CONSTRAINT indexno_pkey PRIMARY KEY (dat_indexno)
)
WITH OIDS;
ALTER TABLE plus.tbl_waldo OWNER TO pg_user;

 

by: ThirtPosted on 2009-10-27 at 08:37:03ID: 25673612

Ya know, I'm wondering if the reason for the double booking problem that just started this week is due to the timezone change. Postgres is running on Linux and I don't think we updated to reflect the new time zone change that kicks in next week, not last week as it use to be.

I just checked the web server that is hosting the application that inserts the records and it's up to date. I'm not a linux person and have to wait on the person to confirm.

 

by: cminearPosted on 2009-10-27 at 08:44:59ID: 25673723

To solve the error you are seeing, add 'timestamp' to the overlap call.  Otherwise, the overlap function doesn't acknowledge that they are time strings and parse them that way.

As to your second query (about whether you really need time zones in the table): you're right; you probably do not need them.  However, whoever designed the database probably thought that they would future-proof the data by recording the time zone; that way, if some other application started either adding or reading data from/for other time zones, it would be ready to cope with that.  As to whether to change it or not, if it's working as is and you really aren't jumping through hoops dealing with it, I would leave it alone.  It's probably not worth the work to determine if changing the table would break anything, or that your data conversion process would be safe.

select dat_indexno, dat_resource from plus.tbl_waldo 
  where dat_resource = 'Conference Room B' 
    and (dat_from,dat_to) overlaps (timestamp '10/27/2009 7:30:00 AM', timestamp '10/27/2009 8:00:00 AM')

                                              
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by: ThirtPosted on 2009-10-27 at 09:04:47ID: 25674036

Excellent, that did the trick. And I agree not to muck around with the data type.

20120131-EE-VQP-002

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