Benjamin Lerer created CASSANDRA-16737:
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Summary: ALTER ... ADD can increase the number of SSTables being
read
Key: CASSANDRA-16737
URL: https://issues.apache.org/jira/browse/CASSANDRA-16737
Project: Cassandra
Issue Type: Bug
Components: CQL/Semantics
Reporter: Benjamin Lerer
Assignee: Benjamin Lerer
With the following SSTables:
{code}
CREATE TABLE my_table (pk int, ck int, v1 int, PRIMARY KEY(pk, ck))
INSERT INTO my_table (pk, ck, v1) VALUES (1, 1, 1) USING TIMESTAMP 1000;
--> flush
INSERT INTO my_table (pk, ck, v1) VALUES (1, 1, 2) USING TIMESTAMP 2000;
--> flush()
INSERT INTO my_table (pk, ck, v1) VALUES (1, 1, 3) USING TIMESTAMP 3000
--> flush()
{code}
the following query:
{code}SELECT pk, ck, v1 FROM my_table WHERE pk = 1 AND ck = 1{code}
will only read the third SSTable.
If we add a column to the table (e.g. {{ALTER TABLE my_table ADD v2 int}}) and
rerun the query, the query will read the 3 SSTables.
The reason for this behavior is due to the fact that C* is trying to read all
the {{fetched}} columns to ensure that it will return a row if at least one of
its column is non null.
In practice for CQL tables, C* does not need to fetch all columns if the row
contains a primary key liveness as it is enough to guaranty that the row
exists. By consequence, even after the addition of the new column C* should
read only the third SSTable.
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