1.17.0 User Documentation for Apache MADlib
Contents

The MADlib sessionize function performs time-oriented session reconstruction on a data set comprising a sequence of events. A defined period of inactivity indicates the end of one session and beginning of the next session. Sessions can be useful in many domains including web analytics [1], network security, manufacturing, finance, and operational analytics.

Function Syntax
sessionize(
source_table,
output_table,
partition_expr,
time_stamp,
max_time,
output_cols,
create_view
)


Arguments

source_table

VARCHAR. Name of the source table that contains the data to be sessionized.

output_table

VARCHAR. Name of the output view or table. (The parameter create_view described below defines whether the output is actually a view or a table.) In addition to the columns in the source_table, the output also contains a new column called session_id:

• session_id=1,2,...n where n is the number of the session in the partition.

partition_expr

VARCHAR. The 'partition_expr' is a single column or a list of comma-separated columns/expressions to divide all rows into groups, or partitions. Sessionization is applied across the rows that fall into the same partition. This parameter can be set to NULL or '' to indicate the sessionization operation is to be applied to the whole input table.

time_stamp

VARCHAR. The time stamp column name that is used for sessionization calculation. Note that the time_stamp column will be sorted in ascending order before the session reconstruction is done within a partition.

max_time

INTERVAL. Maximum delta time (i.e., time out) between subsequent events that define a session. If the elapsed time between subsequent events is longer than max_time, a new session is created.

Note
Note that max_time is of time type INTERVAL which is a PostgreSQL way of describing elapsed time. For more information on INTERVAL please refer to reference [2].
output_cols (optional)

VARCHAR. An optional comma separated list of columns to be written to the output_table. Must be a valid SELECT expression. This is set to '*' by default, which means all columns in the input table will be written to the output_table plus the session_id column. Note that this parameter could include a list containing the partition_expr or any other expressions of interest. E.g., '*, expr1, expr2, etc.' where this means output all columns from the input table plus the expressions listed plus the session_id column.

create_view (optional)
BOOLEAN default: TRUE. Determines whether to create a view or materialize the output as a table. If you only need session info once, creating a view could be significantly faster than materializing as a table. Please note that if you set create_view to NULL (allowed by PostgreSQL) it will get set to the default value of TRUE.

Examples

The data set describes shopper behavior on a notional web site that sells beer and wine. A beacon fires an event to a log file when the shopper visits different pages on the site: landing page, beer selection page, wine selection page, and checkout. Each user is identified by a a user id, and every time a page is visited, the page and time stamp are logged.

Create the data table:

DROP TABLE IF EXISTS eventlog CASCADE;  -- Using CASCADE in case you are running through this example more than once (views used below)
CREATE TABLE eventlog (event_timestamp TIMESTAMP,
user_id INT,
page TEXT,
revenue FLOAT);
INSERT INTO eventlog VALUES
('04/15/2015 02:19:00', 101331, 'CHECKOUT', 16),
('04/15/2015 02:17:00', 202201, 'WINE', 0),
('04/15/2015 03:18:00', 202201, 'BEER', 0),
('04/15/2015 01:03:00', 100821, 'LANDING', 0),
('04/15/2015 01:04:00', 100821, 'WINE', 0),
('04/15/2015 01:05:00', 100821, 'CHECKOUT', 39),
('04/15/2015 02:06:00', 100821, 'WINE', 0),
('04/15/2015 02:09:00', 100821, 'WINE', 0),
('04/15/2015 02:15:00', 101331, 'LANDING', 0),
('04/15/2015 02:16:00', 101331, 'WINE', 0),
('04/15/2015 02:17:00', 101331, 'HELP', 0),
('04/15/2015 02:18:00', 101331, 'WINE', 0),
('04/15/2015 02:29:00', 201881, 'LANDING', 0),
('04/15/2015 02:30:00', 201881, 'BEER', 0),
('04/15/2015 01:05:00', 202201, 'LANDING', 0),
('04/15/2015 01:06:00', 202201, 'HELP', 0),
('04/15/2015 01:09:00', 202201, 'LANDING', 0),
('04/15/2015 02:15:00', 202201, 'WINE', 0),
('04/15/2015 02:16:00', 202201, 'BEER', 0),
('04/15/2015 03:19:00', 202201, 'WINE', 0),
('04/15/2015 03:22:00', 202201, 'CHECKOUT', 21);


Sessionize the table by each user_id:

 DROP VIEW IF EXISTS sessionize_output_view;
'eventlog',             -- Name of input table
'sessionize_output_view',   -- View to store sessionize results
'user_id',             -- Partition input table by user id
'event_timestamp',      -- Time column used to compute sessions
'0:30:0'                -- Use 30 minute time out to define sessions
);
SELECT * FROM sessionize_output_view ORDER BY user_id, event_timestamp;


Result:

   event_timestamp   | user_id |   page   | revenue | session_id
---------------------+---------+----------+---------+------------
2015-04-15 01:03:00 |  100821 | LANDING  |       0 |          1
2015-04-15 01:04:00 |  100821 | WINE     |       0 |          1
2015-04-15 01:05:00 |  100821 | CHECKOUT |      39 |          1
2015-04-15 02:06:00 |  100821 | WINE     |       0 |          2
2015-04-15 02:09:00 |  100821 | WINE     |       0 |          2
2015-04-15 02:15:00 |  101331 | LANDING  |       0 |          1
2015-04-15 02:16:00 |  101331 | WINE     |       0 |          1
2015-04-15 02:17:00 |  101331 | HELP     |       0 |          1
2015-04-15 02:18:00 |  101331 | WINE     |       0 |          1
2015-04-15 02:19:00 |  101331 | CHECKOUT |      16 |          1
2015-04-15 02:29:00 |  201881 | LANDING  |       0 |          1
2015-04-15 02:30:00 |  201881 | BEER     |       0 |          1
2015-04-15 01:05:00 |  202201 | LANDING  |       0 |          1
2015-04-15 01:06:00 |  202201 | HELP     |       0 |          1
2015-04-15 01:09:00 |  202201 | LANDING  |       0 |          1
2015-04-15 02:15:00 |  202201 | WINE     |       0 |          2
2015-04-15 02:16:00 |  202201 | BEER     |       0 |          2
2015-04-15 02:17:00 |  202201 | WINE     |       0 |          2
2015-04-15 03:18:00 |  202201 | BEER     |       0 |          3
2015-04-15 03:19:00 |  202201 | WINE     |       0 |          3
2015-04-15 03:22:00 |  202201 | CHECKOUT |      21 |          3
(21 rows)


Now let's say we want to see 3 minute sessions by a group of users with a certain range of user IDs. To do this, we need to sessionize the table based on a partition expression. Also, we want to persist a table output with a reduced set of columns in the table.

 DROP TABLE IF EXISTS sessionize_output_table;
'eventlog',                    -- Name of input table
'sessionize_output_table',     -- Table to store sessionize results
'user_id < 200000',            -- Partition input table by subset of users
'event_timestamp',             -- Order partitions in input table by time
'180',                         -- Use 180 second time out to define sessions (same as '0:03:0')
'event_timestamp, user_id, user_id < 200000 AS "Department-A1"',    -- Select only user_id and event_timestamp columns, along with the session id as output
'f'                            -- create a table
);
SELECT * FROM sessionize_output_table WHERE "Department-A1"='TRUE' ORDER BY event_timestamp;


Result showing 2 users and 3 total sessions across the group:

   event_timestamp   | user_id | Department-A1 | session_id
---------------------+---------+---------------+------------
2015-04-15 01:03:00 |  100821 | t             |          1
2015-04-15 01:04:00 |  100821 | t             |          1
2015-04-15 01:05:00 |  100821 | t             |          1
2015-04-15 02:06:00 |  100821 | t             |          2
2015-04-15 02:09:00 |  100821 | t             |          2
2015-04-15 02:15:00 |  101331 | t             |          3
2015-04-15 02:16:00 |  101331 | t             |          3
2015-04-15 02:17:00 |  101331 | t             |          3
2015-04-15 02:18:00 |  101331 | t             |          3
2015-04-15 02:19:00 |  101331 | t             |          3
(10 rows)


Literature

NOTE: The following PostgreSQL link refers to documentation resources for the current PostgreSQL database version. Depending upon your database platform version, you may need to change "current" reference in the link to your database version.

If your database platform uses the Greenplum Database (or related variants), please check with the project community and/or your database vendor to identify the PostgreSQL version it is based on.

[1] Sesssions in web analytics https://en.wikipedia.org/wiki/Session_(web_analytics)

[2] PostgreSQL date/time types https://www.postgresql.org/docs/current/static/datatype-datetime.html