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Querying Cassandra with CQL

CQL, Cassandra's language: what the primary key allows, row order, ALLOW FILTERING, writes that overwrite and time to live.

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  • CQL: what is it?

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    Cassandra's query language, whose syntax looks like SQL but which has no join and no subquery.

    A query reads a single table: that is why you create one table per query to serve.

  • Table mesures with PRIMARY KEY ((capteur), instant). Sensor c7 has rows at 2025-02-10, 2025-03-01 and 2025-03-05. What is the first row returned by SELECT * FROM mesures WHERE capteur = 'c7' AND instant >= '2025-03-01' LIMIT 2?

    • The 2025-03-01 row
    • The 2025-03-05 row
    • The 2025-02-10 row
    • None, the query is refused
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    The 2025-03-01 row

    Without ORDER BY, a partition's rows come out in clustering column order, ascending unless the table was created with CLUSTERING ORDER BY. The range removes 10 February, and LIMIT 2 keeps the two left.

  • In PRIMARY KEY ((capteur, jour), instant), the partition key is made of … and the clustering column is ….

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    In PRIMARY KEY ((capteur, jour), instant), the partition key is made of capteur and jour and the clustering column is instant.

    The inner parentheses delimit the partition key; what follows is clustering.

  • Partition key: what is it?

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    The first part of the primary key: its value designates the group of rows, and so the machines, where the data is stored.

    An efficient SELECT always gives the whole partition key, otherwise Cassandra would have to query every node.

  • Reads of one sensor time out and its partition weighs 2 GB, with PRIMARY KEY ((capteur), instant). Which fix is right?

    • Add ALLOW FILTERING to all the reads
    • Put instant alone in the partition key
    • Put the day in the key: ((capteur, jour), instant)
    • Add valeur as a clustering column
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    Put the day in the key: ((capteur, jour), instant)

    The partition key has no time bound: a sensor's partition grows without end. Adding the day cuts it into bounded pieces, one per sensor and per day.

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