DataStax Enterprise 4.0

Creating a URL-encoded prepared statement

The Pig demo examples show the steps required for setting up a prepared CQL query:

Procedure

  1. Format the data

    The example of saving Pig relations from/to Cassandra shows the output schema: the name of the simple_table1 table primary key 'a', represented as a chararray in the relation is paired with a value in the moredata table. In this case, the key for simple_table1 table is only a partitioning key, and only a single tuple is needed.

    The Pig statement to add (moredata) fields to a tuple is:

    grunt> insertformat= FOREACH morevalues GENERATE
             TOTUPLE(TOTUPLE('a',x)),TOTUPLE(y);

    The example of exploring library data works with more complicated data, a partition key and clustering column:

    grunt> insertformat = FOREACH moredata GENERATE
             TOTUPLE(TOTUPLE('a',x),TOTUPLE('b',y),TOTUPLE('c',z)),TOTUPLE(data);
  2. Construct the prepared query

    The output query portion of the cql:// URL is the prepared statement. The prepared statement must be url-encoded to make special characters readable by Pig.

    The example of saving Pig relations from/to Cassandra shows how to construct a prepared query:

    'cql://cql3ks/simple_table1?output_query=UPDATE+cql3ks.simple_table1+set+b+%3D+%3F'

    The key values of the simple_table1 table are automatically transformed into the 'WHERE (key) =' clause to form the output_query portion of a prepared statement.

  3. Execute the query

    To update the simple_table1 table using the values in the simple_table2 (4-6), the prepared statement is executed using these WHERE clauses when the MapReduce job runs:

    ... WHERE a = 5
    ... WHERE a = 4
    ... WHERE a = 6

    This output_query in Pig statement forms the '...' url-encoded portion of the prepared statement:

    grunt> STORE insertformat INTO
             'cql://cql3ks/simple_table1?output_query=UPDATE+cql3ks.simple_table1+set+b+%3D+%3F'
             USING CqlStorage;

    Decoded the UPDATE statement is:

    UPDATE cql3ks.simple_table1 SET b = ?

    The prepared statement represents these queries:

    UPDATE cql3ks.test SET b = 5 WHERE a = 5;
    UPDATE cql3ks.test set b = 4 WHERE a = 4;
    UPDATE cql3ks.test set b = 6 WHERE a = 6;
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