In order to analyze dates and times I am creating a MySQL table where I want to keep the time information. Some example analyses will be stuff like:
Items per day/week/month/year
Items per weekday
Items per hour
etc.
Now in regards to performance, what way should I record in my datatable:
date type: Unix timestamp?
date type: datetime?
or keep date information in one row each, e.g. year, month, day in separate fields?
The last one, for example, would be handy if I'm analysing by weekday; I wouldn't have to perform WEEKDAY(item.date) on MySQL but could simply use WHERE item.weekday = :w.
Based on your usage, you want to use the native datetime format. Unix formats are most useful when the major operations are (1) ordering; (2) taking differences in seconds/minutes/hours/days; and (3) adding seconds/minutes/hours/days. They need to be converted to internal date time formats to get the month or week day, for instance.
You also have a potential indexing issue. If you want to select ranges of days, hours, months and so on for your results, then you want an index on the column. For this purpose an index on a datetime is probably sufficient.
If the summaries are by hour, you might find it helpful to stored the date component in a date field and the hour in a separate column. That would be particularly helpful if you are combining hours from different days.
Whether you break out other components of the date, such as weekday and month, for indexing purposes would depend on the volume of data in the table, performance requirements, and the queries you are planning on running. I would not be inclined to do this, except as a later optimization.
The rule of thumb is: store things as they should be stored, don't do performance tweaks until you're hitting the bottleneck. If you store your date as separate fields, you'll eventually stumble upon a situation you need this date as a whole inside your database (e.g. update query for a particular range of time), and this will be like hell - condition from 3 april 2015 till 15 may 2015 would be as giant as possible.
You should keep your dates as date type. This will grant you maximum flexibility, (most probably) query readability and will keep all of your opportunities to work with them. The only thing I really can recommend is storing the same date divided into year/month/day in next columns - of course, this will bloat your database and require extreme caution on update scenarios, but this will allow you to use any variant of source data in your queries.
There is some value, x, which I am recording every 30 seconds, currently into a database with three fields:
ID
Time
Value
I am then creating a mobile app which will use that data to plot charts in views of:
Last hour
Last 24 hours.
7 Day
30 Day
Year
Obviously, saving every 30 seconds for the last year and then sending that data to a mobile device will be too much (it would mean sending 1051200 values).
My second thought was perhaps I could use the average function in MySQL, for example, collect all of the averages for every 7 days (creating 52 points for a year), and send those points. This would work, but still MySQL would be trawling through creating averages and if many users connect, it's going to be bad.
So simply put, if these are my views, then I do not need to keep track of all that data. Nobody should care what x was a year ago to the precision of every 30 seconds, this is fine. I should be able to use "triggers" to create some averages.
I'm looking for someone to check what I have below is reasonable:
Store values every 30s in a table (this will be used for the hour view, 120 points)
When there are 120 rows are in the 30s table (120 * 30s = 60 mins = 1 hour), use a trigger to store the first half an hour in a "half hour average" table, remove the first 60 entries from the 30s table. This new table will need to have an id, start time, end time and value. This half hour average will be used for the 24 hour view (48 data points).
When the half hour table has more than 24 entries (12 hours), store the first 6 as an average in a 6 hour average table and then remove from the table. This 6 hour average will be used for the 7 day view (28 data points).
When there are 8 entries in the 6 hour table, remove the first 4 and store this as an average day, to be used in the 30 day view (30 data points).
When there are 14 entries in the day view, remove the first 7 and store in a week table, this will be used for the year view.
This doesn't seem like the best way to me, as it seems to be more complicated than I would imagine it should be.
The alternative is to keep all of the data and let mysql find averages as and when needed. This will create a monstrously huge database. I have no idea about the performance yet. The id is an int, time is a datetime and value is a float. Is 1051200 records too many? Now is a good time to add, I would like to run this on a raspberry pi, but if not.. I do have my main machine which I could use.
Your proposed design looks OK. Perhaps there are more elegant ways of doing this, but your proposal should work too.
RRD (http://en.wikipedia.org/wiki/Round-Robin_Database) is a specialised database designed to do all of this automatically, and it should be much more performant than MySQL for this specialised purpose.
An alternative is the following: keep only the original table (1051200 records), but have a trigger that generates the last hour/day/year etc views every time a new record is added (e.g. every 30 seconds) and store/cache the result somewhere. Then your number-crunching workload is independent of the number of requests/clients you have to serve.
1051200 records may or may not be too many. Test in your Raspberry Pi to find out.
Let me give a suggestion on the physical layout of your table, regardless on whether you decide to keep all data or "prune" it from time to time...
Since you generate a new row "every 30 seconds", then Time can serve as a natural key without fear of exceeding the resolution of the underlying data type and causing duplicated keys. You don't need ID in this scenario1, so your table is simply:
Time (PK)
Value
And since InnoDB tables are clustered, not having secondary indexes2 means the whole table is stored in a single B-Tree, which is as efficient as it gets from storage and querying perspective. On top of that, Value is automatically covered, which may not have been the case in your original design unless you specifically designed your index(es) for that.
Using time as key can be tricky in general, but I think may be worth it in this particular case.
1 Unless there are other tables that reference it through FOREIGN KEYs, or you have already written too much code that depends on it.
2 Which would be necessary in the original design to support efficient aggregation.
I'm making an air conditioning scheduler website for school. A user will be able to add a temperature and humidity setting for any of the 30 minute intervals throughout the day, for seven days of the week. For example, a user will be able to say that on Sunday, at 3:30 PM, they want the cooler (rather than the heater) to cool their home down to 70 degrees and a humidity index of 50 for 15 minutes. I could use advice setting up a MySQL table (or tables) to handle such commands. It's not the individual variables for all the potential settings I'm worried about, but rather handling all those times for all seven days.
So far I am thinking of having one big table called Scheduler which would handle the entire week. The day AND time slots for the seven days of the week could go into a VARCHAR column called time_slot, and would have both the day and the time slot in military time. For example.
time_slot (a VARCHAR column)
sunday_0000 (this is sunday at midnight)
.....
sunday_1630 (this is sunday at 4:30 pm)
.....
sunday_1130 (this is the final possible sunday time slot at 11:30 PM)
monday_0000 (this is the start of monday)
(continue for all seven days)
the remaining columns for the table would be all the necessary settings a user could put, as well as a duration from 30 seconds to the full 30 minutes before the next potential time slot. Does anyone have any ideas for a more efficient MySQL table? Perhaps something that gives each individual day it's own table?
You may want to consider having multiple columns, using TINYINT for day (1-7) and TIME (00:00-23:59). This way one could set the time for each days individually or all at once.
e.g.
UPDATE scheduler
set ...
where TIME = '12:00';
We're building an app that stores "hours of operation" for various businesses. What is the easiest way to represent this data so you can easily check if an item is open?
Some options:
Segment out blocks (every 15 minutes) that you can mark "open/closed". Checking involves seeing if the "open" bit is set for the desired time (a bit like a train schedule).
Storing a list of time ranges (11am-2pm, 5-7pm, etc.) and checking whether the current time falls in any specified range (this is what our brain does when parsing the strings above).
Does anyone have experience in storing and querying timetable information and any advice to give?
(There's all sorts of crazy corner cases like "closed the first Tuesday of the month", but we'll leave that for another day).
store each contiguous block of time as a start time and a duration; this makes it easier to check when the hours cross date boundaries
if you're certain that hours of operation will never cross date boundaries (i.e. there will never be an open-all-night sale or 72-hour marathon event et al) then start/end times will suffice
The most flexible solution might be use the bitset approach. There are 168 hours in a week, so there are 672 15-minute periods. That's only 84 bytes worth of space, which should be tolerable.
I'd use a table like this:
BusinessID | weekDay | OpenTime | CloseTime
---------------------------------------------
1 1 9 13
1 2 5 18
1 3 5 18
1 4 5 18
1 5 5 18
1 6 5 18
1 7 5 18
Here, we have a business that has regular hours of 5 to 6, but shorter hours on sunday.
A query for if open would be (psuedo-sql)
SELECT #isOpen = CAST
(SELECT 1 FROM tblHours
WHERE BusinessId = #id AND weekDay = #Day
AND CONVERT(Currentime to 24 hour) IS BETWEEN(OpenTime,CloseTime)) AS BIT;
If you need to store edge cases, then just have 365 entries, one per day...its really not that much in the grand scheme of things, place an index on the day column and businessId column.
Don't forget to store the businesses timezone in a separate table (normalize!), and perform a transform between your time and it before making these comparisons.
OK, I'll throw in on this for what it's worth.
I need to handle quite a few things.
Fast / Performant Query
Any increments of time, 9:01 PM, 12:14, etc.
International (?) - not sure if this is an issue even with timezones, at least in my case but someone more versed here feel free to chime in
Open - Close spanning to the next day (open at noon, close at 2:00 AM)
Multiple timespans / day
Ability to override specific days (holidays, whatever)
Ability for overrides to be recurring
Ability to query for any point in time and get businesses open (now, future time, past time)
Ability to easily exclude results of businesses closing soon (filter businesses closing in 30 minutes, you don't want to make your users 'that guy that shows up 5 minutes before closing in the food/beverage industry)
I like a lot of the approaches presented and I'm borrowing from a few of them. In my website, project, whatever I need to take into consideration I may have millions of businesses and a few of the approaches here don't seem to scale well to me personally.
Here's what I propose for an algorithm and structure.
We have to make some concrete assumptions, across the globe, anywhere, any time:
There are 7 days in a week.
There are 1440 minutes in one day.
There are a finite number of permutations of minutes of open / closed that are possible.
Not concrete but decent assumptions:
Many permutations of open/closed minutes will be shared across businesses reducing total permutations actually stored.
There was a time in my life I could easily calculate the actual possible combinations to this approach but if someone could assist/thinks it would be useful, that would be great.
I propose 3 tables:
Before you stop reading, consider in the real-world 2 of these tables will be small enough cache neatly. This approach isn't going to be for everyone either due to the sheer complexity of code required to interpret a UI to the data model and back again if needed. Your mileage and needs may vary. This is an attempt at a reasonable 'enterprise' level solution, whatever that means.
HoursOfOperations Table
ID | OPEN (minute of day) | CLOSE (minute of day)
1 | 360 | 1020 (example: 9 AM - 5 PM)
2 | 365 | 1021 (example: edge-case 9:05 AM - 5:01 PM (weirdos) )
etc.
HoursOfOperations doesn't care about what days, just open and close and uniqueness. There can be only a single entry per open/close combination. Now, depending on your environment either this entire table can be cached or it could be cached for the current hour of the day, etc. At any rate, you shouldn't need to query this table for every operation. Depending on your storage solution I envision every column in this table as indexed for performance. As time progresses, this table likely has an exponentially inverse likelihood of INSERT(s). Really though, dealing with this table should mostly be an in-process operation (RAM).
Business2HoursMap
Note: In my example I'm storing "Day" as a bit-flag field/column. This is largely due to my needs and the advancement of LINQ / Flags Enums in C#. There's nothing stopping you from expanding this to 7 bit fields. Both approaches should be relatively similar in both storage logic and query approach.
Another Note: I'm not entering into a semantics argument on "every table needs a PK ID column", please find another forum for that.
BusinessID | HoursID | Day (or, if you prefer split into: BIT Monday, BIT Tuesday, ...)
1 | 1 | 1111111 (this business is open 9-5 every day of the week)
2 | 2 | 1111110 (this business is open 9:05 - 5:01 M-Sat (Monday = day 1)
The reason this is easy to query is that we can always determine quite easily the MOTD (Minute of the Day) that we're after. If I want to know what's open at 5 PM tomorrow I grab all HoursOfOperations IDS WHERE Close >= 1020. Unless I'm looking for a time range, Open becomes insignificant. If you don't want to show businesses closing in the next half-hour, just adjust your incoming time accordingly (search for 5:30 PM (1050), not 5:00 PM (1020).
The second query would naturally be 'give me all business with HoursID IN (1, 2, 3, 4, 5), etc. This should probably raise a red flag as there are limitations to this approach. However, if someone can answer the actual permutations question above we may be able to pull the red flag down. Consider we only need the possible permutations on any one side of the equation at one time, either open or close.
Considering we've got our first table cached, that's a quick operation. Second operation is querying this potentially large-row table but we're searching very small (SMALLINT) hopefully indexed columns.
Now, you may be seeing the complexity on the code side of things. I'm targeting mostly bars in my particular project so it's going to be very safe to assume that I will have a considerable number of businesses with hours such as "11:00 AM - 2:00 AM (the next day)". That would indeed be 2 entries into both the HoursOfOperations table as well as the Business2HoursMap table. E.g. a bar that is open from 11:00 AM - 2:00 AM will have 2 references to the HoursOfOperations table 660 - 1440 (11:00 AM - Midnight) and 0 - 120 (Midnight - 2:00 AM). Those references would be reflected into the actual days in the Business2HoursMap table as 2 entries in our simplistic case, 1 entry = all days Hours reference #1, another all days reference #2. Hope that makes sense, it's been a long day.
Overriding on special days / holidays / whatever.
Overrides are by nature, date based, not day of week based. I think this is where some of the approaches try to shove the proverbial round peg into a square hole. We need another table.
HoursID | BusinessID | Day | Month | Year
1 | 2 | 1 | 1 | NULL
This can certainly get more complex if you needed something like "on every second Tuesday, this company goes fishing for 4 hours". However, what this will allow us to do quite easily is allow 1 - overrides, 2 - reasonable recurring overrides. E.G. if year IS NULL, then every year on New Years day this weirdo bar is open from 9:00 AM to 5:00 PM keeping in line with our above data examples. I.e. - If year were set, it's only for 2013. If month is null, it's every first day of the month. Again, this won't handle every scheduling scenario by NULL columns alone, but theoretically, you could handle just about anything by relying on a long sequence of absolute dates if needed.
Again, I would cache this table on a rolling day basis. I just can't realistically see the rows for this table in a single-day snapshot being very large, at least for my needs. I would check this table first as it is well, an override and would save a query against the much larger Business2HoursMap table on the storage-side.
Interesting problem. I'm really surprised this is the first time I've really needed to think this through. As always, very keen on different insights, approaches or flaws in my approach.
I think I'd personally go for a start + end time, as it would make everything more flexible. A good question would be: what's the chance that the block size would change at a certain point? Then pick the solution that best fits your situation (if it's liable to change I'd go for the timespans definately).
You could store them as a timespan, and use segments in your application. That way you have the easy input using blocks, while keeping the flexibility to change in your datastore.
To add to what Johnathan Holland said, I would allow for multiple entries for the same day.
I would also allow for decimal time, or another column for minutes.
Why? many restaurants and some businesses, and many businesses around the world have lunch and or afternoon breaks. Also, many restaurants (2 that I know of near my house close at odd non-15-increments time. One closes at 9:40 PM on Sundays, and one closes at 1:40 AM.
There is also the issue of holiday hours , such as stores closing early on thanksgiving day, for example, so you need to have calendar-based override.
Perhaps what can be done is a date/time open, date-time close, such as this:
businessID | datetime | type
==========================================
1 10/1/2008 10:30:00 AM 1
1 10/1/2008 02:45:00 PM 0
1 10/1/2008 05:15:00 PM 1
1 10/2/2008 02:00:00 AM 0
1 10/2/2008 10:30:00 AM 1
etc. (type: 1 being open and 0 closed)
And have all the days in the coming 1 or two years precalculated 1-2 years in advance. Note that you would only have 3 columns: int, date/time/bit so the data consumption should be minimal.
This will also allow you to modify specific dates for odd hours for special days, as they become known.
It also takes care of crossing over midnight, as well as 12/24 hour conversions.
It is also timezone agnostic. If you store start time and duration, when you calculate the end time, is your machine going to give you the TZ adjusted time? Is that what you want? More code.
as far as querying for open-closed status: query the date-time in question,
select top 1 type from thehours where datetimefield<=somedatetime and businessID = somebusinessid order by datetime desc
then look at "type". if one, it's open, if 0, it's closed.
PS: I was in retail for 10 years. So I am familiar with the small business crazy-hours problems.
The segment blocks are better, just make sure you give the user an easy way to set them. Click and drag is good.
Any other system (like ranges) is going to be really annoying when you cross the midnight boundary.
As for how you store them, in C++ bitfields would probably be best. In most other languages, and array might be better (lots of wasted space, but would run faster and be easier to comprehend).
I would think a little about those edge-cases right now, because they are going to inform whether you have a base configuration plus overlay or complete static storage of opening times or whatever.
There are so many exceptions - and on a regular basis (like snow days, irregular holidays like Easter, Good Friday), that if this is expected to be a reliable representation of reality (as opposed to a good guess), you'll need to address it pretty soon in the architecture.
How about something like this:
Store Hours Table
Business_id (int)
Start_Time (time)
End_Time (time)
Condition varchar/string
Open bit
'Condition' is a lambda expression (text for a 'where' clause). Build the query dynamically. So for a particular business you select all of the open/close times
Let Query1 = select count(open) from store_hours where #t between start_time and end_time and open = true and business_id = #id and (.. dynamically built expression)
Let Query2 = select count(closed) from store_hours where #t between start_time and end_time and open = false and business_id = #id and (.. dynamically built expression)
So end the end you want something like:
select cast(Query1 as bit) & ~cast(Query2 as bit)
If the result of the last query is 1 then the store is open at time t, otherwise it is closed.
Now you just need a friendly interface that can generate your where clauses (lambda expressions) for you.
The only other corner case that I can think of is what happens if a store is open from say 7am to 2am on one date but closes at 11pm on the following date. Your system should be able to handle that as well by smartly splitting up the times between the two days.
There is surely no need to conserve memory here, but perhaps a need for clean and comprehensible code. "Bit twiddling" is not, IMHO, the way to go.
We need a set container here, which holds any number of unique items and can determine quickly and easily whether an item is a member or not. The setup reuires care, but in routine use a single line of simply understood code determines if you are open or closed
Concept:
Assign index number to every 15 min block, starting at, say, midnight sunday.
Initialize:
Insert into a set the index number of every 15 min block when you are open. ( Assuming you are open fewer hours than you are closed. )
Use:
Subtract from interesting time, in minutes, midnight the previous sunday and divide by 15. If this number is present in the set, you are open.