Small question regarding Cassandra please.
I have a row, I can see it in Cassandra, I can query it etc.
The row looks like this.
CREATE TABLE student_Registration(
Id int PRIMARY KEY,
Name text,
Event text
);
Id Name Event
101 Ashish Ninza
I would like to ask, is there a way to know when this row got inserted please?
I tried looking up for some command to get the insert time, similar to SELECT TTL (Name) from, but for "when the row got inserted" and no luck so far.
May I ask what would be the best way to know when the row got inserted please?
Thank you
That's the responsibility of the writetime function (doc):
SELECT writetime(Name) from student_Registration
Update: But you need to keep in mind following:
the writetime shows the update time of the individual cell, and they could be updated separately
Write time could be setup explicitly when writing the data
Related
Hi I am having a cassandra table. my table has around 200 records in it . later i have altered the table to add a new column named budget which is of type boolean . I want to set the default value to be true for that column . what should be the cql looks like.
I am trying the following command but it didnt work
cqlsh:Openmind> update mep_primecastaccount set budget = true ;
SyntaxException: line 1:46 mismatched input ';' expecting K_WHERE
appreciate any help
thank you
Any operation that would require a cluster wide read before write is not supported (as it wont work in the scale that Cassandra is designed for). You must provide a partition and clustering key for an update statement. If theres only 200 records a quick python script or can do this for you. Do a SELECT * FROM mep_primecastaccount and iterate through ResultSet. For each row issue an update. If you have a lot more records you might wanna use spark or hadoop but for a small table like that a quick script can do it.
Chris's answer is correct - there is no efficient or reliable way to modify a column value for each and every row in the database. But for a 200-row table that doesn't change in parallel, it's actually very easy to do.
But there's another way that can work also on a table of billions of rows:
You can handle the notion of a "default value" in your client code. Pre-existing codes will not have a value for "budget" at all: It won't be neither true, nor false, but rather it will be outright missing (a.k.a. "null"). You client code may, when it reads a row with a missing "budget" value, replace it by some default value of its choice - e.g., "true".
I have a table in Cassandra say employee(id, email, role, name, password) with only id as my primary key.
I want to ...
1. Add another column (manager_id) in with a default value in it
I know that I can add a column in the table but there is no way i can provide a default value to that column through CQL. I can also not update the value for manager_id later since I need to know the id (Partition key and the values are randomly generated unique values which i don't know) to update the row. Is there any way I can achieve this?
2. Rename this table to all_employee.
I also know that its not allowed to rename a table in cassandra. So I am trying to copy the data of table(employee) to csv and copy from csv to new table (all_employee) and deleting the old table(employee). I am doing this through an automated script with cql queries in it and script works fine but will fail if it gets executed again(Which i can not restrict) since the table employee will not be there once its deleted. Essentially I am looking for "If exists" clause in COPY query which is not supported in cql. Is there any other way I can achieve the outcome?
Please note that the amount of data in the table is very small so performance in not an issue.
For #1
I dont think cassandra support default column . You need to do that from your appliaction. Write some default value every time you insert a row.
For #2
you can check if the table exists before trying to copy from it.
SELECT your_table_name FROM system_schema.tables WHERE keyspace_name='your_keyspace_name';
Trying to learn how cassandra works here, was very hard to get insights about how data is stored behind the scenes, hence im not sure i got it right and wanted to get corrected if i got it wrong, so far i understand that a typical row that the partition key (leftmost column or compuond column in the primary key) is unique, a row would be written to disk in a chunk that looks like this :
where the columns are sorted by their names.
But, if the partition key is not unique for then it would be considered a "wide row" and a row would look like the following examples :
Please correct me if i got it wrong...
For Second part where you have compound primary key the structure will be somthing similar to first except for the fact that:
ColumnName will be replaced by usrname1.comments|username1.comments_ts|username2.comments|username2.comments_ts
VideoId : usrname1.comments|username1.comments_ts|username2.comments|username2.comments_ts
Same thing will be true for comments_by_user
username: videoid1.comments|videoid1.comments_ts|videoid2.comments|videoid2.comments_ts
PS: I am not good at drawing images so you have to do with this text answer.
For more details Refer Slide 48
I hava created a cassandra table with 20 million records. Now I want to delete the expired data decided by one none primary key column. But it doesn't support the operation on the column. So I try to retrieve the table and get the data line by line to delete the data.Unfortunately,it is too huge to retrieve. Otherwise,I couldn't delete the whole table, how could I achieve my goal?
Your question is actually, how to get the data from the table in bulks (also called pagination).
You can do that by selecting different slices from your primary key: For example, if your primary key is some sort of ID, select a range of IDs each time, process the results and do whatever you want to do with them, then get the next range, and so on.
Another way, which depends on the driver you're working with, will be to use fetch_size. You can see a Python example here and a Java example here.
How do we retrieve a row in Cassandra using Astyanax?
I have a web application which requires pagination to be done on the server side, the db is cassandra. The row key is a UUID and I have few columns within a row, so I am trying to do pagination on the row keys.
I have put together a solution with which i am not completely happy. The issue is that when i do my first search based on the search filter i get from UI, I don't know the UUID of the first row, So I prepare a Query which gives me first 6 records, I store the key of the 6th record in a MAP and put it in a session, so when user from UI request for a second page, i retrieve this key(UUID) and take it as a start for the next set of records to be retrieved. I was trying to find a cleaner approach.
EDIT in response to question changes...
In that case, sounds like you are doing it just fine. Also sounds like you are using OOP then as otherwise the rows are not in order. While playOrm's solution is more elegant returning you a cursor that you store in the session, I think what you have is just fine.
EDIT since our code changed
line 74 in this link is how we do it(you can drill down into that cursor class basically to see what we do).
https://github.com/deanhiller/playorm/blob/master/src/main/java/com/alvazan/orm/layer9z/spi/db/cassandra/CassandraSession.java
which if you have 10 row keys you just pass in the list of keys.
I am not sure what you mean by pagination???? Are you saying you have a from and to row key and want the rows between them? Are you using order the cluster by row key then?
Another completely different direction to go for ordering is playOrm though which can do S-SQL and if things are partitioned, you can do joins and other such(that link above is to one of the files in playOrm actually).
I am really not sure what you mean "you don't have the row key with you"
later,
Dean