It is possible to use Datastore.key to generate a new key for an element:
const taskKey = datastore.key(['Task', 'sampleTask']);
When running a query, we may get an endCursor that can be used to get the next results.
The cursor is some base64 encoded token that contains the project ID, kind, and key of the last element fetched, with a little bit of unknown binary data.
Would there be a way/method to get that base64 cursor value given the key of an item and kind+project ID?
As previously answered, it's not possible to infer a cursor from a key because a cursor is tied to a query. You can however, filter your queries by key. E.g. select * from Task where key > Key(Task, 'sampleTask')
No, it's not possible.
Or I should rather say that it doesn't make much sense to (attempt to) obtain a cursor from an entity by itself because a cursor only has meaning in the context of the original query from which it was obtained. From Limitations of cursors (emphasis mine):
Cursors are subject to the following limitations:
A cursor can be used only by the same application that performed the original query, and only to continue the same query. To use the cursor
in a subsequent retrieval operation, you must reconstitute the
original query exactly, including the same entity kind, ancestor
filter, property filters, and sort orders. It is not possible to
retrieve results using a cursor without setting up the same query from
which it was originally generated.
Also from Cursors and data updates:
The cursor's position is defined as the location in the result list
after the last result returned. A cursor is not a relative position in
the list (it's not an offset); it's a marker to which Cloud Datastore
can jump when starting an index scan for results.
Related
I have a table with a composite key; there is both a partition and a sort key. I know that the java sdk allows me to query by just the partition key. However, if I do this then the docs say I will get this iterator back ItemCollection<QueryOutcome>. This means for me to work with this data, I will have to iterate over the entire collection in order to fulfill my needs.
It would be easier if I was able to get back a Map<T, V> type where the key here would be the sort key. That way, I can quickly find rows for a particular sort key. Is this possible? I would rather not iterate over the collection just to find certain items with a certain sort key value.
If you just want an item with a certain sort key, that’s a get item. Don’t do a Query.
You may be confused by DynamoDB’s use of the word Query. That’s not the only way to query the database. It’s one way to query which happens to have the name Query.
I've been reading a DynamoDB docs and was unable to understand if it does make sense to query on Global Secondary Index with a usage of 'contains' operator.
My problem is as follows: my dynamoDB document has a list of embedded objects, every object has a 'code' field which is unique:
{
"entities":[
{"code":"entity1Code", "name":"entity1Name"},
{"code":"entity2Code", "name":"entity2Name"}
]
}
I want to be able to get all documents that contain entities with entity.code = X.
For this purpose I'm considering adding a Global Secondary Index that would contain all entity.codes that are present in current db document separated by a comma. So the example above would look like:
{
"entities":[
{"code":"entity1Code", "name":"entity1Name"},
{"code":"entity2Code", "name":"entity2Name"}
],
"entitiesGlobalSecondaryIndex":"entityCode1,entityCode2"
}
And then I would like to apply filter expression on entitiesGlobalSecondaryIndex something like: entitiesGlobalSecondaryIndex contains entityCode1.
Would this be efficient or using global secondary index does not make sense in this way and DynamoDB will simply check the condition against every document which is similar so scan?
Any help is very appreciated,
Thanks
The contains operator of a query cannot be run on a partition Key. In order for a query to use any sort of operators (contains, begins with, > < ect...) you must have a range attributes- aka your Sort Key.
You can very well set up a GSI with some value as your PK and this code as your SK. However, GSIs are replication of the table - there is a slight potential for the data ina GSI to lag behind that of the master copy. If the query you're doing against this GSI isn't very often, then you're probably safe from that.
However. If you are trying to do this to the entire table at once then it's no better than a scan.
If what you need is a specific Code to return all its documents at once, then you could do a GSI with that as the PK. If you add a date field as the SK of this GSI it would even be time sorted. If you query against that code in that index, you'll get every single one of them.
Since you may have multiple codes, if they aren't too many per document, you maybe could use a Sparse Index - if you have an entity with code "AAAA" then you also have an attribute named AAAA (or AAAAflag or something.) It is always null/does not exist Unless the entities contains that code. If you do a GSI on this AAAflag attribute, it will only contain documents that contain that entity code, and ignore all where this attribute does not exist on a given document. This may work for you if you can also provide a good PK on this to keep the numbers well partitioned and if you don't have too many codes.
Filter expressions by the way are different than all of the above. Filter expressions are run on tbe data that would be returned, after it is already read out of the table. This is useful I'd you have a multi access pattern setup, but don't want a particular call to get all the documents associated with a particular PK - in the interests of keeping the data your code is working with concise. The query with a filter expression still retrieves everything from that query, but only presents what makes it past the filter.
If are only querying against a particular PK at any given time and you want to know if it contains any entities of x, then a Filter expressions would work perfectly. Of course, this is only per PK and not for your entire table.
If all you need is numbers, then you could do a count attribute on the document, or a meta document on that partition that contains these values and could be queried directly.
Lastly, and I have no idea if this would work or not, if your entities attribute is a map type you might very well be able to filter against entities code - and maybe even with entities.code.contains(value) if it was an SK - but I do not know if this is possible or not
I want to return a node where the node has a property as a specific uuid and I just want to return one of them (there could be several matches).
g.V().where('application_uuid', eq(application_uuid).next()
Would the above query return all the nodes? How do I just return 1?
I also want to get the property map of this node. How would I do this?
You would just do:
g.V().has('application_uuid', application_uuid).next()
but even better would be the signature that includes the vertex label (if you can):
g.V().has('vlabel', 'application_uuid', application_uuid).next()
Perhaps going a bit further if you explicitly need just one you could:
g.V().has('vlabel', 'application_uuid', application_uuid).limit(1).next()
so that both the graph provider and/or Gremlin Server know your intent is to only next() back one result. In that way, you may save some extra network traffic/processing.
This is a very basic query. You should read more about gremlin. I can suggest Practical Gremlin book.
As for your query, you can use has to filter by property, and limit to get specific number of results:
g.V().has('application_uuid', application_uuid).limit(1).next()
Running your query without the limit will also return a single result since the query result is an iterator. Using toList() will return all results in an array.
Have to list all the records from a DynamoDB table, without any filter expression.
I want to limit the number of records hence using DynamoDBScanExpression with setLimit.
DynamoDBScanExpression scanExpression = new DynamoDBScanExpression();
....
// Set ExclusiveStartKey
....
scanExpression.setLimit(10);
However, the scan operation returns more than 10 results always !!!!
Is this the expected behaviour and if so how?
Python Answer
It is not possible to set a limit for scan() operations, however, it is possible to do so with a query.
A query searches through items, the rows in the database. It starts at the top or bottom of the list and finds items based on set criteria. You must have a partion and a sort key to do this.
A scan on the other hand searches through the ENTIRE database and not by items, and, as a result, is NOT ordered.
Since queries are based on items and scan is based on the ENTIRE database, only queries can support limits.
To answer OP's question, essentially it doesn't work because you're using scan not query.
Here is an example of how to use it using CLIENT syntax. (More advanced syntax version. Sorry I don't have a simpler example that uses resource. you can google that.)
def retrieve_latest_item(self):
result = self.dynamodb_client.query(
TableName="cleaning_company_employees",
KeyConditionExpression= "works_night_shift = :value",
ExpressionAttributeValues={':value': {"BOOL":"True"}},
ScanIndexForward = False,
Limit = 3
)
return result
Here is the DynamoDB module docs
I have a few objects created on my database and I need to delete some of the repeating attributes related to them.
The query I'm trying to run is:
UPDATE gemp1_product objects REMOVE ingredients[1] WHERE (r_object_id = '08015abd8002cd68')
But all I get is the folloing error message:
Error querying databse.
[DM_QUERY_E_UPDATE_INDEX]error: "UPDATE: Unable to REMOVE tghe attribute ingredients at index 1."
[DM_OBJECT_W_DELETE_ATTR_POSITION_ERROR]warning: "attempt to delete
non-existent attribute 88"
Object 08015abd8002cd68 exists and I can see it on the database. Queries like SELECT and DELETE work fine but I do not want to delete the whole object.
There is no easy way to do this. The reason is that repeating attributes are ordered, to enable multiple repeating attributes to be synchronized for a given object.
Either
set the attribute value to be empty for the given position, and change your code to discard empty attributes, or
use multiple DQL statements to shuffle the order so that the last one becomes empty, or
change your data model, e.g. use a single attribute as a property bag with pre-defined delimiters.
Details (1)
UPDATE gemp1_product OBJECTS SET ingredients[1] = '' WHERE ...
Details (2)
For each index; first find the value of index+1:
SELECT ingredients
FROM gemp1_product
WHERE (i_position*-1)-1 = <index+1>
ENABLE (ROW_BASED)
Use the value in a new query:
UPDATE gemp1_product OBJECTS SET ingredients[1] = '<value_from_above>' WHERE ...
It should also be possible to do this by nesting DQL somehow, but it might not be worth the effort.
Something is either wrong with your query or with your repository. I think you are mistyping your attribute name or using wrong index in your UPDATE query.
If you google for DM_OBJECT_W_DELETE_ATTR_POSITION_ERROR you'll see on this link a bit more detailed explanation:
CAUSE: Program executed a DeleteAttr operation that specified an non-existent attribute position (either a negative number or a number larger than the number of attributes in the object).
From this you could guess that type isn't in consistent state, or that you are trying to remove too big index of your repeating attribute, etc. Did you checked your repository with Consistency checker Job and other similar Jobs?
As of for the removing of repeating property (sttribute) value with DQL query, this is unachievable with single query since you need to specify index position which you don't know at first. But writing a simple script or doing it manually if it's not big amount of values to delete is the way you want to go.