I have an issue in ydn db.
I need to fetch records by ids. Because it is not possible yet to say something like
"WHERE id IN (10, 11, 12)" you have to fetch id to id.
The problem comes here... I do not have to oppurtunity to sort fetched data by date with ydn db.
Now the question: Currently I sort the records with my own algorithm.
Is a indexeddb wrapper out there which is able to fetch records like my example above? If not so, is there a library which sorts an array of objects by given object key? (In my case it would be the date)
Thanks in advance. :-)
It is not possible. Not all sorted query are possible in IndexedDB. In this case, you have to resolve to in-memory sorting or using temporary object store. There is incomplete implementation ydn.db.Buffer to do this.
Related
I have the following structure in my DB:
MessagesInvolvingUserID:
- [UserID01]:
- [MessageID01]: [TimeStamp01]
- [MessageID02]: [TimeStamp02]
...
- [UserID02]:
- [MessageID03]: [TimeStamp03]
- [MessageID04]: [TimeStamp04]
...
I'm wondering how to efficiently construct a Dictionary representing the latest messageID (based on timestamp value) for each UserID ?
I think a "brute force" approach would work:
observe MessagesInvolvingUserID for value
loop over every snapshot the filter the latest message ID
construct the Dictionary
But that seems awfully inefficient since that involves downloading and doing the work client side. Any suggestion for a more elegant approach ?
Why don’t you make a Timestamp key under each message with timestamp value and then query messages sorting them by their timestamps? What language are you using to query items from the database? Have you tried something like:
ref.child(“MessagesInvolvingUserID”).child(userID).child("MessageID01").queryOrderedByChild("timestamp").observeEventType(.ChildAdded, withBlock: { snapshot in ...
If you want to have a list that shows the latest timestamp for each user, you should store a list that shows the latest timestamp for each user in your database.
LatestTimstampForUserID:
- [UserID01]: [TimeStamp02]
- [UserID02]: [TimeStamp04]
...
Now you can very efficiently read this data from the database and show the list.
The cost is that you'll need to update multiple locations in the database when a new message is posted for a user. This is very common when using NoSQL databases, such as the Firebase Realtime Database.
As part of migrating from SQL to DynamoDB I am trying to create a DynamoDB table. The UI allows users to search based on 4 attributes start date, end date, name of event and source of event.
The table has 6 attributes and the above four are subset of it with other attributes being priority and location. The query as described above makes it mandatory to search based on the above four values. whats the best way to store the information in DynamoDB that will help me in querying based on start date and end date fairly easy.
I thought of creating a GSI with hashkey as startdate, rangekey as end date and GSI on the rest two attributes ?
Inshort:
My table in DynamoDB will have 6 attributes
EventName, Location, StartDate, EndDate, Priority and source.
Query will have 4 mandatory attributes
StartDate, EndDate, Source and Event Name.
Thanks for the help.
You can use greater than/less than comparison operators as part of your query http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/QueryAndScan.html
So you could try to build a table with schema:
(EventName (hashKey), "StartDate-EndDate" (sortKey), other attributes)
In this case the sort-key is basically a combination of start and end date allowing you to use >= (on the first part) and <= (on the second part)... dynamodb uses ASCII based alphabetical ordering... so lets assume your sortKey looks like the following: "73644-75223" you could use >= "73000-" AND <= "73000-76000" to get the given event.
Additionally, you could create a GSI on your table for each of your remaining attributes that need to be read via query. You then could project data into your index that you want to fetch with the query. In contrast to LSI, queries from GSI do not fetch attributes that are not projected. Be aware of the additional costs (read/write) involved by using GSI (and LSI)... and the additional memory required by data projections...
Hope it helps.
Simple thing. While inserting data to table A I have a HashKey id and additional hash index for column ex_id, which is kind of a foreign key in table B.
When inserting a new data into table A I would like to create an exception whenever data is inserted with value in column ex_id that doesn't have a correspondent entry in table B.
I thought that ConditionExpression is the way to go, but can't make it work - probably missing something obvious. Tried to use contains()...
Any ideas?
As per my knowledge this would not be possible at DynamoDB end because there are no relationship between the tables.
What you can do is that you can have a condition at the application level, which checks on its own and throw an exception before inserting the value in table A. (You can query table B for that "Id" if found then insert else throw exception)
DynamoDB does not natively support any kind of foreign key support, everything works on a per table basis, per key basis. DynamoDB's approach is to handle such logic at the client level. For example see the dynamodb transactions client. This library allows you to perform transactions across tables which either all succeed or all rollback.
For your case, I would first make a getItem request to table B (use consistent read) if it exists then write to table A.
Then I would enable streams on table A and write a lambda function to check if any data violations get written to the table.
I'd like to use flyway for a DB update with the situation that an DB already exists with productive data in it. The problem I'm looking at now (and I did not find a nice solution yet), is the following:
There is an existing DB table with numeric IDs, e.g.
create table objects ( obj_id number, ...)
There is a sequence "obj_seq" to allocate new obj_ids
During my DB migration I need to introduce a few new objects, hence I need new
object IDs. However I do not know at development time, what ID
numbers these will be
There is a DB trigger which later references these IDs. To improve performance I'd like to avoid determine the actual IDs every time the trigger runs but rather put the IDs directly into the trigger
Example (very simplified) of what I have in mind:
insert into objects (obj_id, ...) values (obj_seq.nextval, ...)
select obj_seq.currval from dual
-> store this in variable "newID"
create trigger on some_other_table
when new.id = newID
...
Now, is it possible to dynamically determine/use such variables? I have seen the flyway placeholders but my understanding is that I cannot set them dynamically as in the example above.
I could use a Java-based migration script and do whatever string magic I like - so, that would be a way of doing it, but maybe there is a more elegant way using SQL?
Many thx!!
tge
If the table you are updating contains only reference data, get rid of the sequence and assign the IDs manually.
If it contains a mix of reference and user data, you need to select the id based on values in other columns.
i have two tables
asset employee
assetid-pk empid-pk
empid-fk
now, i have a form to populate the asset table but it cant because of the foreign key constraint..
what to do?
thx
Tk
Foreign keys are created for a good reason - to prevent orphan rows at a minimum. Create the corresponding parent and then use the appropriate value as the foreign key value on the child table.
You should think about this update as a series of SQL statements, not just one statement. You'll process the statements in order of dependency, see example.
Asset
PK AssetID
AssetName
FK EmployeeID
etc...
Employee
PK EmployeeID
EmployeeName
etc...
If you want to "add" a new asset, you'll first need to know which employee it will be assigned to. If it will be assigned to a new employee, you'll need to add them first.
Here is an example of adding a asset named 'BOOK' for a new employee named 'Zach'.
DECLARE #EmployeeFK AS INT;
INSERT (EmployeeName) VALUES ('Zach') INTO EMPLOYEE;
SELECT #EmployeeFK = ##IDENTITY;
INSERT (AssetName, EmployeeID) VALUES ('BOOK',#EmployeeFK) INTO ASSET;
The important thing to notice above, is that we grab the new identity (aka: EmployeeID) assigned to 'Zach', so we can use it when we add the new asset.
If I understand you correctly, are you trying to build the data graph locally before persisting to the data? That is, create the parent and child records within the application and persist it all at once?
There are a couple approaches to this. One approach people take is to use GUIDs as the unique identifiers for the data. That way you don't need to get the next ID from the database, you can just create the graph locally and persist the whole thing. There's been a debate on this approach between software and database for a long time, because while it makes a lot of sense in many ways (hit the database less often, maintain relationships before persisting, uniquely identify data across systems) it turns out to be a significant resource hit on the database.
Another approach is to use an ORM that will handle the persistence mapping for you. Something like NHibernate, for example. You would create your parent object and the child objects would just be properties on that. They wouldn't have any concept of foreign keys and IDs and such, they'd just be objects in code related by being set as properties on each other (such as a "blog post" object with a generic collection of "comment" objects, etc.). This graph would be handed off to the ORM which would use its knowledge of the mapping between the objects and the persistence to send it off to the database in the correct order, perhaps giving back the same object but with ID numbers populated.
Or is this not what you're asking? It's a little unclear, to be honest.