Wide-column stores

The wide-column model on Cassandra: keyspace, table, partition key and clustering columns, one table per query, and how it differs from the column storage of data warehouses.

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Wide-column store: what is it?
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A distributed database where rows are spread across machines according to part of their identifier, then stored in sorted order inside each group.
Reading a whole group of neighbouring rows is very fast, but a query that does not say which group to look in forces every machine to be queried.
With this table, in which order are the rows of sensor c7's partition stored?

```sql
CREATE TABLE mesures (
capteur text,
instant timestamp,
valeur double,
PRIMARY KEY ((capteur), instant)
);
`
- By instant ascending
- By valeur ascending, from the smallest reading to the largest
- In insertion order, with no particular sort
- By capteur, then by valeur
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By instant ascending
The clustering column instant sets the sort inside the partition, ascending by default. valeur is not part of the key: no sort applies to it.
In Cassandra, a … groups tables and sets their … factor.
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In Cassandra, a keyspace groups tables and sets their replication factor.
The replication factor says how many nodes each partition is copied to. A keyspace thus plays a role close to a schema, with replication added.
Partition key: what is it?
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The part of the primary key whose value, passed through a hash function, decides which cluster nodes a row is stored on.
All rows sharing the same value form a partition, stored together. An efficient query always supplies this value in full.
Reads for one sensor time out and its partition weighs 2 GB. The partition key is capteur alone. What is the fix?
- Add an extra clustering column at the end of the key
- Raise the keyspace's replication factor
- Add the day to the partition key: ((capteur, jour), instant)
- Query with ALLOW FILTERING so the server scans fewer rows
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Add the day to the partition key: ((capteur, jour), instant)
As long as the partition key holds no time bound, a sensor's partition grows endlessly and cannot be spread. A day in the key cuts it into bounded pieces. An extra clustering column or more copies does not change its size.

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