PostgreSQL 9.3 – How to Get Records for Yesterday’s Date

postgresql-9.3

Currently my application is running over 2 dbs one is snapshotDB one is evercamDB. In snapshotDB we are saving records now it want to retrieve them , currently am using this query

select camera_id,count(*) as snapshot_count 
from snapshots 
where snapshot_id in (select snapshot_id 
                      from snapshots 
                      where created_at = 'now'::date - 1) 
group by camera_id

what i want is to get records from snapshots such as am considering today is 2016-01-21 and i want to get all records from yesterday. '2016-01-20 00:00:00' to '2016-01-20 23:59:59'. i really dont know how to bind time with 'now'::date - 1 in such format as i have written.

And i want to place this date and time in (select snapshot_id from snapshots where created_at = 'now'::date - 1) so that i can get records as i want.

Any suggestion or help will be appreciated

Best Answer

You need to cast created_at as a date - that will remove the time.

now() also contains a time, so you need to use current_date

And finally your IN (..) is not needed, you can apply that condition directly in the where clause:

select camera_id, count(*) as snapshot_count 
from snapshots 
where created_at::date = current_date - 1
group by camera_id;

Note that the above will not use an index on created_at. If the performance is not acceptable and the condition would benefit from using an index, you can use something like this:

select camera_id, count(*) as snapshot_count 
from snapshots 
where created_at >= date_trunc('day', current_timestamp) - interval '1' day
  and create_at < date_trunc('day', current_timestamp)
group by camera_id;

Window function?

A window function (count(*) over ()) does not seem to be what you want, since you don't have unaggregated rows.
You could add to the inner subquery:

count(*) OVER ()

.. to get the count of distinct landing_path_id, which is one other possible number that might be of interest. But that doesn't seem to be what you meant by "the total number of rows from that records select".
Or you could add to the inner subquery:

sum(count(*)) OVER ()

.. to get the total count with every landing_path_id redundantly, but that would seem pointless. Just mentioning that to demonstrate it's possible to run a window function over the result of an aggregate function in a single pass. Details for that:

Updated question

Your result, just without total_count in the records subquery. Now accounting for the LIMIT in the inner SELECT. Even though a maximum of 10 distinct landing_path_id is selected, all qualifying landing_path_id are counted.

To get both in one scan and reuse count and sum separately I introduce a CTE:

WITH cte AS (
  SELECT sum(entrances) AS entrances
       , count(*) over () AS total_count
  FROM   report_la
  WHERE  profile_id = 3777614
  GROUP  BY landing_path_id
  LIMIT  10
  )
SELECT row_to_json(selected_records)::text AS data
FROM  (   
   SELECT (SELECT total_count FROM cte LIMIT 1) AS total_count
        , array_to_json(array_agg(row_to_json(records))) AS data
   FROM  (SELECT entrances FROM cte) records
   ) selected_records;

If you don't care about the attribute name, you can have that cheaper with a subquery:

SELECT row_to_json(selected_records)::text AS data
FROM  (   
   SELECT min(total_count) AS total_count
        , array_to_json(array_agg(row_to_json(ROW(entrances)))) AS data
   FROM (
      SELECT sum(entrances) AS entrances
           , count(*) over () AS total_count  -- shouldn't show up in result
      FROM   report_la
      WHERE  profile_id = 3777614
      GROUP  BY landing_path_id
      LIMIT  1
      ) records
   ) selected_records;

You get the default attribute name f1 instead of entrances, since the ROW expression does not preserve the column name.

If you need a certain attribute name, you could cast the row to a registered type. (Ab-)using a TEMP TABLE to register my row type for the session:

CREATE TEMP TABLE rec1 (entrances bigint);

...
        , array_to_json(array_agg(row_to_json(ROW(entrances)::rec1))) AS data
...

This would be a bit faster than the CTE. Or, more verbose but without cast:

...
        , array_to_json(array_agg(row_to_json(
                   (SELECT x FROM (SELECT records.entrances) x)))) AS data
...

Detailed explanation in this related answer:

Select columns inside json_agg

SQL Fiddle.

PostgreSQL – How to Select the First or Last Entry for a Specific Day

You can't use an aggregate function in a condition, you need to use a sub-select

SELECT EntryID 
FROM AttendanceREcords 
WHERE StaffID = 'xxxxx' 
AND ArrivalTime = (select min(ArrivalTime) 
                   from AttendanceREcords
                   where StaffID = 'xxxxx');

But this can be done more efficiently by using a LIMIT clause:

SELECT EntryID 
FROM AttendanceREcords 
WHERE StaffID = 'xxxxx' 
ORDER BY arrivalTime 
LIMIT 1

There is a difference between the two statements: if more than one row has the same minimum arrival time, the first one will return all of them, the second one only one row.

Another alternative that is usually more efficient that a sub-select is using a window function:

SELECT EntryID
FROM (
  SELECT EntryID, 
         dense_rank() over (order by arrivalTime) as rnk
  FROM AttendanceREcords 
  WHERE StaffID = 'xxxxx' 
) t
where rnk = 1;

By changing the order by arrivalTime you can select the first or the last.

If you want the first and last in a single query you can do something like this:

SELECT EntryID,
       arrivalTime
FROM (
  SELECT EntryID, 
         min(arrivalTime) as min_time,
         min(arrivalTime) as max_time,
  FROM AttendanceREcords 
  WHERE StaffID = 'xxxxx' 
) t
where arrival_time = min_time
   or arrival_time = max_time;

Best Answer

Related Solutions

PostgreSQL – Return Total Number of Rows and Selected Aggregated Data

Window function?

Updated question

PostgreSQL – How to Select the First or Last Entry for a Specific Day

Related Question