Quick Date/Time Index Question

On Sep 22, 2011, at 2:13 PM, David Johnston wrote:

Hey,

On 9.0.4

I have a database field that stores a timestamp to second+ precision; however, I want to search against it only to day precision. If I leave the field in second precision and try to “WHERE field BETWEEN date0 AND date0” I get no results (OK, fine) but then I cast the field to date “WHERE field::date BETWEEN date0 AND date0” and get the expected results. So now I want to index “field::date” by I cannot create a functional index on “field::date” OR “CAST(field AS date)” OR “date_trunc(‘day’,field)” due to either syntax (::) or non-IMMUTABLE function errors (cast; date_trunc).

Is there some other way to create an index on only the “date” portion of the field? Is it even necessary since any index ordered on timestamp is also, by definition, order on date as well?

It seems to me that it wouldn't be necessarily, but maybe there are some cases where if you had just the date in the index and could therefore test for equality instead of range, it would allow the planner to do more efficient plans. Maybe.

I suspect in most cases, the performance differences would be negligible.

On Sep 22, 2011, at 17:13, David Johnston wrote:

Is there some other way to create an index on only the "date" portion of the
field?

The cast from timestamptz to date is time zone dependent, which is why it's volatile and can't be used indexes. However, you can do this:

test=# create table timestamps (ts timestamp with time zone primary key);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "timestamps_pkey" for table "timestamps"
CREATE TABLE
test=# create index ts_date_idx on timestamps (cast(ts AS DATE));
ERROR: functions in index expression must be marked IMMUTABLE
test=# create index ts_date_idx on timestamps (cast(ts at time zone 'UTC' AS DATE));
CREATE INDEX

Just decide what time zone you want the index to be in.

Is it even necessary since any index ordered on timestamp is also,
by definition, order on date as well?

Another option is to use some other operator rather than =, if you're selecting items, such as BETWEEN or >= and <, such as

WHERE field >= date0 AND field < date0 + 1

This would use an index on the timestamp column.

Michael Glaesemann
grzm seespotcode net

"David Johnston" <polobo@yahoo.com> wrote:

I have a database field that stores a timestamp to second+ precision;
however, I want to search against it only to day precision. If I leave the
field in second precision and try to "WHERE field BETWEEN date0 AND date0" I
get no results (OK, fine) but then I cast the field to date "WHERE
field::date BETWEEN date0 AND date0" and get the expected results. So now I
want to index "field::date" by I cannot create a functional index on
"field::date" OR "CAST(field AS date)" OR "date_trunc('day',field)" due to
either syntax (::) or non-IMMUTABLE function errors (cast; date_trunc).

Is there some other way to create an index on only the "date" portion of the
field? Is it even necessary since any index ordered on timestamp is also,
by definition, order on date as well?

Not necessarily a direct answer, but have you tried querying
"WHERE field >= date0 AND field < (date0 + 1)"?

Tim

"David Johnston" <polobo@yahoo.com> writes:

I have a database field that stores a timestamp to second+ precision;
however, I want to search against it only to day precision. If I leave the
field in second precision and try to "WHERE field BETWEEN date0 AND date0" I
get no results (OK, fine) but then I cast the field to date "WHERE
field::date BETWEEN date0 AND date0" and get the expected results.

Try "WHERE field BETWEEN date0 AND date0+1". When comparing a date to a
timestamp, the date is considered to represent midnight of its day, so
you're testing for a zero-width range there.

regards, tom lane

Tom Lane <tgl@sss.pgh.pa.us> wrote:

I have a database field that stores a timestamp to second+ precision;
however, I want to search against it only to day precision. If I leave the
field in second precision and try to "WHERE field BETWEEN date0 AND date0" I
get no results (OK, fine) but then I cast the field to date "WHERE
field::date BETWEEN date0 AND date0" and get the expected results.

Try "WHERE field BETWEEN date0 AND date0+1". When comparing a date to a
timestamp, the date is considered to represent midnight of its day, so
you're testing for a zero-width range there.

Dare I to say it? :-) Not quite true:

| tim=# SELECT t
| tim-# FROM (VALUES ('2011-01-01 00:00:00'::TIMESTAMP),
| tim(# ('2011-01-02 00:00:00'::TIMESTAMP)) AS d (t)
| tim-# WHERE t BETWEEN '2011-01-01'::DATE AND ('2011-01-01'::DATE + 1);
| t
| ---------------------
| 2011-01-01 00:00:00
| 2011-01-02 00:00:00
| (2 Zeilen)

| tim=#

So you would have to assert that no timestamp will ever fall
on midnight.

Tim