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From "Durity, Sean R" <SEAN_R_DUR...@homedepot.com>
Subject RE: [EXTERNAL] n00b q re UPDATE v. INSERT in CQL
Date Fri, 25 Oct 2019 19:07:25 GMT
Everything in Cassandra is an insert. So, an update and an insert are functionally equivalent.
An update doesn't go update the existing data on disk; it is a new write of the columns involved.
So, the difference in your scenario is that with the "targeted" update, you are writing less
of the columns again.

So, instead of inserting 10 values (for example), you are inserting 3 (pk1, pk2, and col1).
This would mean less disk space used for your data cleanup. Once Cassandra runs its compaction
across your data (with a simplifying assumption that all data gets compacted), there would
be no disk space difference for the final result.

I would do the updates, if the size/scope of the data involved is significant.


Sean Durity – Staff Systems Engineer, Cassandra

-----Original Message-----
From: James A. Robinson <jim.robinson@gmail.com>
Sent: Friday, October 25, 2019 10:49 AM
To: user@cassandra.apache.org
Subject: [EXTERNAL] n00b q re UPDATE v. INSERT in CQL

Hi folks,

I'm working on a clean-up task for some bad data in a cassandra db.
The bad data in this case are values with mixed case that will need to
be lowercased.  In some tables the value that needs to be changed is a
primary key, in other cases it is not.

From the reading I've done, the situations where I need to change a
primary key column to lowercase will mean I need to perform an INSERT
of the entire row using the new primary key values merged with the old
non-primary-key values, followed by a DELETE of the old primary key
row.

My question is, on a table where I need to update a column that isn't
primary key, should I perform a limited UPDATE in that situation like
I would in SQL:

UPDATE ks.table SET col1 = ? WHERE pk1 = ? AND pk2 = ?

or will there be any downsides to that over an INSERT where I specify
all columns?

INSERT INTO ks.table (pk1, pk2, col1, col2, ...) VALUES (?,?,?,?, ...)

In SQL I'd never question just using the update but my impression
reading the blogosphere is that Cassandra has subtleties that I might
not be grasping when it comes to UPDATE v. INSERT behavior...

Jim

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