Nadav Har'El created CASSANDRA-14478:
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Summary: Improve the documentation of UPDATE vs INSERT
Key: CASSANDRA-14478
URL: https://issues.apache.org/jira/browse/CASSANDRA-14478
Project: Cassandra
Issue Type: Improvement
Components: Documentation and Website
Reporter: Nadav Har'El
New Cassandra users often wonder about the difference between the INSERT and
UPDATE cql commands when applied to ordinary data (not counters or
transactions). Usually, they are told them that there is really no difference
between the two - both of them can insert a new row or update an existing one.
The Cassandra CQL documentation
[http://cassandra.apache.org/doc/latest/cql/dml.html#update|http://cassandra.apache.org/doc/latest/cql/dml.html#update,]
is fairly silent on the question - on the one hand it doesn't explicitly say
they are the same, but on the other hand describes them both as doing the same
things, and doesn't explicitly mention any difference.
But there is an important difference, which was raised in the past in
CASSANDRA-11805: INSERT adds a row marker, while UPDATE does not. What does
this mean? Basically an UPDATE requests that individual cells of the row be
added, but not that the row itself be added; So if one later deletes the same
individual cells with DELETE, the entire row goes away. However, an "INSERT"
not only adds the cells, it also requests that the row be added (this is
implemented via a "row marker"). So if later all the row's individual cells are
deleted, an empty row remains behind (i.e., the primary of the row which now
has no content is still remembered in the table).
I'm not sure what is the best way to explain this, but what I wrote in the
paragraph above is a start.
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