Contents

• SuperLogLog Estimator
  • Contents of the extension
  • Usage
  • Problems

SuperLogLog Estimator

This is an implementation of SuperLogLog algorithm as described in the paper "LogLog Counting of Large Cardinalities," published by Flajolet and Durand in 2003. Generally it is an improved LogLog estimator, with truncation and restriction rules.

Contents of the extension

The extension provides the following elements

• superloglog_estimator data type (may be used for columns, in PL/pgSQL)

• functions to work with the superloglog_estimator data type

  • superloglog_size(error_rate real)

  • superloglog_init(error_rate real)

  • superloglog_add_item(counter superloglog_estimator, item anyelement)

  • superloglog_get_estimate(counter superloglog_estimator)

  • superloglog_reset(counter superloglog_estimator)

  • length(counter superloglog_estimator)

  The purpose of the functions is quite obvious from the names, alternatively consult the SQL script for more details.

• aggregate functions

  • superloglog_distinct(anyelement, real)
  • superloglog_distinct(anyelement)

  where the 1-parameter version uses default error rate 2.5%. That's quite generous and it may result in unnecessarily large estimators, so if you can work with worse error rate, pass the parameter explicitly.

Usage

Using the aggregate is quite straightforward - just use it like a regular aggregate function

db=# SELECT superloglog_distinct(i, 0.01)
     FROM generate_series(1,100000) s(i);

and you can use it from a PL/pgSQL (or another PL) like this:

DO LANGUAGE plpgsql $$
DECLARE
    v_counter superloglog_estimator := superloglog_init(0.01);
    v_estimate real;
BEGIN
    PERFORM superloglog_add_item(v_counter, 1);
    PERFORM superloglog_add_item(v_counter, 2);
    PERFORM superloglog_add_item(v_counter, 3);

    SELECT superloglog_get_estimate(v_counter) INTO v_estimate;

    RAISE NOTICE 'estimate = %',v_estimate;
END$$;

And this can be done from a trigger (updating an estimate stored in a table).

Problems

Be careful about the implementation, as the estimators may easily occupy several kilobytes (depends on the precision etc.). Keep in mind that the PostgreSQL MVCC works so that it creates a copy of the row on update, an that may easily lead to bloat. So group the updates or something like that.

This is of course made worse by using unnecessarily large estimators, so always tune the estimator to use the lowest amount of memory.