Ordering problem with varchar (DESC)

am Wed, dem 31.01.2007, um 10:46:17 -0500 mailte Alexandre Leclerc folgendes:

Hi all,

We have a column (varchar) that has plain text time and it is indexed.

How can I fix that so that the result is exactly like the first one but
perfectly reversed in it's order?

Use the right data-typ for your data, in this case TIMESTAMP. Then you
can order this data likewise reverse and get a performance boost by the
way...

Andreas
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Andreas Kretschmer
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Alexandre Leclerc wrote:

Hi all,

We have a column (varchar) that has plain text time and it is indexed.
When I do a query with the index, all the data is in the right order,
but when I user ORDER BY .. DESC, the order is messed up. Example:

By index 1: (date, time, data)
SELECT * from t1;
date (date type) time (varchar) data
2007-01-17 8h40 d1
2007-01-30 9h30 d2
2007-01-30 12h00 d3
2007-01-30 13h45 d4
2007-01-30 17h20 d5

SELECT * from t1 ORDER BY date, time DESC;
date (date type) time (varchar) data
2007-01-30 9h30 d2
2007-01-30 17h20 d5
2007-01-30 13h45
2007-01-30 12h00 d3
2007-01-17 8h40 d1

I don't know why, this is like if the 'time' varchar was trimmed then
used for the ordering.

How can I fix that so that the result is exactly like the first one but
perfectly reversed in it's order?

The ordering of the result from the first query is incidental of the
order the rows are returned by the index, not of the actual values
returned. As to the second query, it's ordering correctly as the values
in your time field are sorted as strings. In the ideal scenario you'd
change the datatype of your time field. If for some reason that's not
possible try something along these lines might work:

SELECT *
FROM t1
ORDER BY (date || ' ' || replace(time, 'h', ':'))::timestamp;

As others have said, VARCHAR is the incorrect data type to be using
here. You should either be using INTERVAL or TIMESTAMP depending on
what you want. You can even combine date and time into a single
TIMESTAMP field. Only use VARCHAR when no other data type will do.

"SELECT * from t1;" is not an ordered query and any consistency of order
is coincidental (typically it comes out in the same order it went in,
but there's no guarantee of that).

Try "SELECT * from t1 ORDER BY date, time;", and I suspect you will get:
date (date type) time (varchar) data
2007-01-17 8h40 d1
2007-01-30 12h00 d3
2007-01-30 13h45 d4
2007-01-30 17h20 d5
2007-01-30 9h30 d2

To use your current schema, you need to zero-fill your hours, so 9h30
needs to be 09h30 and so forth.

--
Brandon Aiken
CS/IT Systems Engineer
-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of Alexandre
Leclerc
Sent: Wednesday, January 31, 2007 10:46 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Ordering problem with varchar (DESC)

Hi all,

We have a column (varchar) that has plain text time and it is indexed.
When I do a query with the index, all the data is in the right order,
but when I user ORDER BY .. DESC, the order is messed up. Example:

By index 1: (date, time, data)
SELECT * from t1;
date (date type) time (varchar) data
2007-01-17 8h40 d1
2007-01-30 9h30 d2
2007-01-30 12h00 d3
2007-01-30 13h45 d4
2007-01-30 17h20 d5

SELECT * from t1 ORDER BY date, time DESC;
date (date type) time (varchar) data
2007-01-30 9h30 d2
2007-01-30 17h20 d5
2007-01-30 13h45 d4
2007-01-30 12h00 d3
2007-01-17 8h40 d1

I don't know why, this is like if the 'time' varchar was trimmed then
used for the ordering.

How can I fix that so that the result is exactly like the first one but
perfectly reversed in it's order?

Best regards.

--
Alexandre Leclerc

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Brandon Aiken a �crit :

As others have said, VARCHAR is the incorrect data type to be using
here. You should either be using INTERVAL or TIMESTAMP depending on
what you want. You can even combine date and time into a single
TIMESTAMP field. Only use VARCHAR when no other data type will do.

I dearly would like to do that, but it is impossible (because of the
software/technology that uses the database). I would have use a
TIMESTAMP for that.

Try "SELECT * from t1 ORDER BY date, time;", and I suspect you will get:
date (date type) time (varchar) data
2007-01-17 8h40 d1
2007-01-30 12h00 d3
2007-01-30 13h45 d4
2007-01-30 17h20 d5
2007-01-30 9h30 d2

To use your current schema, you need to zero-fill your hours, so 9h30
needs to be 09h30 and so forth.

Exactly. This is sorted that way. This is what I'll do, inserting a 0.

Best regards.

--
Alexandre Leclerc

Daniel Verite a ï¿œcrit :

Alexandre Leclerc wrote:

SELECT * from t1 ORDER BY date, time DESC;
date (date type) time (varchar) data
2007-01-30 9h30 d2
2007-01-30 17h20 d5
2007-01-30 13h45 d4
2007-01-30 12h00 d3
2007-01-17 8h40 d1

I don't know why, this is like if the 'time' varchar was trimmed then
used for the ordering.

How can I fix that so that the result is exactly like the first one but
perfectly reversed in it's order?

I believe ORDER BY date, replace(time,'h',':')::time DESC would work.

That worked perfectly. Unfortunately I can't control the sql query in
the situation I am in. But... I know this is the white space that does
the issue.

Or just use directly a time datatype instead of varchar, or only one datetime
column instead of the two, and order by that column.

Or use a leading '0' instead of a leading space when the hour is less than 10...

Yep, this is the only solution that will work for that situation right
now: inserting a leading '0' instead of a white space.

Thank you for your help.
Best regards.

--
Alexandre Leclerc