1.16 User Documentation for Apache MADlib
Contents

The goal of the MADlib path function is to perform regular pattern matching over a sequence of rows, and to extract useful information about the pattern matches. The useful information could be a simple count of matches or something more involved like aggregations or window functions.

Symbols are used to identify particular rows of interest. Then, standard PostgreSQL pattern matching using symbols can be applied to identify patterns across the rows of interest. (This is similar in concept to regular expressions which match patterns within strings of text.)

For example, a symbol can be defined for purchase events by on-line shoppers. Then, preceding events that led to the purchase can be identified and operated on, perhaps to find the common actions that resulted in a purchase. Or conversely, to find actions that resulted in an exit without a purchase having been made.

Steps on how to use path functions:

1. Partition input rows.
2. Order the partitions.
3. Define symbols to match rows of interest.
4. Define regular expression of symbols and operators to define patterns to match in your ordered partitions.
5. Define an aggregate function to compute for each pattern match.
6. If desired, output the pattern matches for inspection or to operate on them with subsequent queries.

Function Syntax
path(
source_table,
output_table,
partition_expr,
order_expr,
symbol,
pattern,
aggregate_func,
persist_rows,
overlapping_patterns
)


Arguments

source_table

VARCHAR. Name of the source table, containing data for path analysis.

output_table

VARCHAR. Name of the result table.

partition_expr

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

order_expr

VARCHAR. This expression controls the order in which rows are processed or matched in a partition. For example, time is a common way to order partitions.

symbol

VARCHAR. Symbols enable you to express patterns of interest in a simple way (see definition of ‘pattern’ argument below). A symbol identifies a row of a particular type that you’re searching for as part of a pattern match. Symbol definition uses the standard PostgreSQL assignment statement 'identifier := expression;' [1]. A given row can only match one symbol. If a row matches multiple symbols, the symbol that comes first in the symbol definition list will take precedence.

pattern

VARCHAR. The 'pattern' clause defines the pattern that the path algorithm searches for. You express the pattern using symbols and operators following regular PostgreSQL pattern matching syntax and rules [2].

Note
Symbols defined using more than one (1) character need to be enclosed in parentheses '()' when referenced in the 'pattern' argument. For example:
• a symbol defined as 'a' in the 'symbol' argument can be used directly in the 'pattern' argument
• a symbol defined as 'abc' in the 'symbol' argument must be written as '(abc)' in the 'pattern' argument

The following pattern matching metacharacters are supported:

• | denotes alternation (either of two alternatives).
• ? denotes repetition of the previous item zero or one time.
• * denotes repetition of the previous item zero or more times.
• + denotes repetition of the previous item one or more times.
• {m} denotes repetition of the previous item exactly m times.
• {m,} denotes repetition of the previous item m or more times.
• {m,n} denotes repetition of the previous item at least m and not more than n times.
• Parentheses () can be used to group items into a single logical item.

aggregate_func (optional)

VARCHAR, default NULL. A comma-separated list of aggregates to be applied to the pattern matches [3]. You can think of this input parameter as being like a SELECT clause. Please note that window functions cannot currently be used in the parameter 'aggregate_func'. If you want to use a window function [4], output the pattern matches and write a SQL query with a window function over the output tuples (see 'persist_rows' parameter below).

If you just want to output the pattern matched rows and not compute any aggregates, you can put NULL or '' in the 'aggregate_func' parameter.

persist_rows (optional)

BOOLEAN, default FALSE. If TRUE the matched rows are persisted in a separate output table. This table is named as <output_table>_tuples (the string "_tuples" is added as suffix to the value of output_table).

overlapping_patterns (optional)

BOOLEAN, default FALSE. If TRUE find every occurrence of the pattern in the partition, regardless of whether it might have been part of a previously found match.

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. Other pages on the site like help pages show up in the logs as well. Let’s assume that the log has been sessionized.

1. Create the date table:
DROP TABLE IF EXISTS eventlog;
CREATE TABLE eventlog (event_timestamp TIMESTAMP,
user_id INT,
session_id INT,
page TEXT,
revenue FLOAT);
INSERT INTO eventlog VALUES
('04/15/2015 01:03:00', 100821, 100, 'LANDING', 0),
('04/15/2015 01:04:00', 100821, 100, 'WINE', 0),
('04/15/2015 01:05:00', 100821, 100, 'CHECKOUT', 39),
('04/15/2015 02:06:00', 100821, 101, 'WINE', 0),
('04/15/2015 02:09:00', 100821, 101, 'WINE', 0),
('04/15/2015 01:15:00', 101121, 102, 'LANDING', 0),
('04/15/2015 01:16:00', 101121, 102, 'WINE', 0),
('04/15/2015 01:17:00', 101121, 102, 'CHECKOUT', 15),
('04/15/2015 01:18:00', 101121, 102, 'LANDING', 0),
('04/15/2015 01:19:00', 101121, 102, 'HELP', 0),
('04/15/2015 01:21:00', 101121, 102, 'WINE', 0),
('04/15/2015 01:22:00', 101121, 102, 'CHECKOUT', 23),
('04/15/2015 02:15:00', 101331, 103, 'LANDING', 0),
('04/15/2015 02:16:00', 101331, 103, 'WINE', 0),
('04/15/2015 02:17:00', 101331, 103, 'HELP', 0),
('04/15/2015 02:18:00', 101331, 103, 'WINE', 0),
('04/15/2015 02:19:00', 101331, 103, 'CHECKOUT', 16),
('04/15/2015 02:22:00', 101443, 104, 'BEER', 0),
('04/15/2015 02:25:00', 101443, 104, 'CHECKOUT', 12),
('04/15/2015 02:29:00', 101881, 105, 'LANDING', 0),
('04/15/2015 02:30:00', 101881, 105, 'BEER', 0),
('04/15/2015 01:05:00', 102201, 106, 'LANDING', 0),
('04/15/2015 01:06:00', 102201, 106, 'HELP', 0),
('04/15/2015 01:09:00', 102201, 106, 'LANDING', 0),
('04/15/2015 02:15:00', 102201, 107, 'WINE', 0),
('04/15/2015 02:16:00', 102201, 107, 'BEER', 0),
('04/15/2015 02:17:00', 102201, 107, 'WINE', 0),
('04/15/2015 02:18:00', 102871, 108, 'BEER', 0),
('04/15/2015 02:19:00', 102871, 108, 'WINE', 0),
('04/15/2015 02:22:00', 102871, 108, 'CHECKOUT', 21),
('04/15/2015 02:25:00', 102871, 108, 'LANDING', 0),
('04/15/2015 02:17:00', 103711, 109, 'BEER', 0),
('04/15/2015 02:18:00', 103711, 109, 'LANDING', 0),
('04/15/2015 02:19:00', 103711, 109, 'WINE', 0);

2. Calculate the revenue by checkout:
DROP TABLE IF EXISTS path_output, path_output_tuples;
'eventlog',                -- Name of input table
'path_output',             -- Table name to store path results
'session_id',              -- Partition input table by session
'event_timestamp ASC',     -- Order partitions in input table by time
'buy:=page=''CHECKOUT''',  -- Define a symbol for checkout events
'sum(revenue) as checkout_rev',    -- Aggregate:  sum revenue by checkout
TRUE                       -- Persist matches
);
SELECT * FROM path_output ORDER BY session_id, match_id;

Result:
 session_id | match_id | checkout_rev
------------+----------+--------------
100 |        1 |           39
102 |        1 |           15
102 |        2 |           23
103 |        1 |           16
104 |        1 |           12
108 |        1 |           21
(6 rows)

Note that there are 2 checkouts within session 102, which is apparent from the 'match_id' column. This serves to illustrate that the 'aggregate_func' operates on a per pattern match basis, not on a per partition basis. If in fact we wanted revenue by partition ('session_id' in this example), then we could do:
SELECT session_id, sum(checkout_rev) FROM path_output GROUP BY session_id ORDER BY session_id;

Result:
 session_id | sum
------------+-----
100 |  39
102 |  38
103 |  16
104 |  12
108 |  21
(5 rows)

Since we set TRUE for 'persist_rows', we can view the associated pattern matches:
SELECT * FROM path_output_tuples ORDER BY session_id ASC, event_timestamp ASC;

Result:
   event_timestamp   | user_id | session_id |   page   | revenue | symbol | match_id
---------------------+---------+------------+----------+---------+--------+----------
2015-04-15 01:05:00 |  100821 |        100 | CHECKOUT |      39 | buy    |        1
2015-04-15 01:17:00 |  101121 |        102 | CHECKOUT |      15 | buy    |        1
2015-04-15 01:22:00 |  101121 |        102 | CHECKOUT |      23 | buy    |        2
2015-04-15 02:19:00 |  101331 |        103 | CHECKOUT |      16 | buy    |        1
2015-04-15 02:25:00 |  101443 |        104 | CHECKOUT |      12 | buy    |        1
2015-04-15 02:22:00 |  102871 |        108 | CHECKOUT |      21 | buy    |        1
(6 rows)

Notice that the 'symbol' and 'match_id' columns are added to the right of the matched rows.
3. We are interested in sessions with an order placed within 4 pages of entering the shopping site via the landing page. We represent this by the regular expression: '(land)[^(land)(buy)]{0,2}(buy)'. In other words, visit to the landing page followed by from 0 to 2 non-entry, non-sale pages, followed by a purchase. The SQL is as follows:
DROP TABLE IF EXISTS path_output, path_output_tuples;
'eventlog',                -- Name of input table
'path_output',             -- Table name to store path results
'session_id',              -- Partition input table by session
'event_timestamp ASC',     -- Order partitions in input table by time
'land:=page=''LANDING'',
wine:=page=''WINE'',
beer:=page=''BEER'',
other:=page<>''LANDING'' AND page<>''WINE'' AND page<>''BEER'' AND  page<>''CHECKOUT''',    -- Symbols for  page types
'sum(revenue) as checkout_rev',    -- Aggregate:  sum revenue by checkout
TRUE                       -- Persist matches
);
SELECT * FROM path_output ORDER BY session_id, match_id;

Result:
 session_id | match_id | session_rev
------------+----------+-------------
100 |        1 |          39
102 |        1 |          15
102 |        2 |          23
(3 rows)

Now view the associated pattern matches:
SELECT * FROM path_output_tuples ORDER BY session_id ASC, event_timestamp ASC;

Result:
   event_timestamp   | user_id | session_id |   page   | revenue | symbol | match_id
---------------------+---------+------------+----------+---------+--------+----------
2015-04-15 01:03:00 |  100821 |        100 | LANDING  |       0 | land   |        1
2015-04-15 01:04:00 |  100821 |        100 | WINE     |       0 | wine   |        1
2015-04-15 01:05:00 |  100821 |        100 | CHECKOUT |      39 | buy    |        1
2015-04-15 01:15:00 |  101121 |        102 | LANDING  |       0 | land   |        1
2015-04-15 01:16:00 |  101121 |        102 | WINE     |       0 | wine   |        1
2015-04-15 01:17:00 |  101121 |        102 | CHECKOUT |      15 | buy    |        1
2015-04-15 01:18:00 |  101121 |        102 | LANDING  |       0 | land   |        2
2015-04-15 01:19:00 |  101121 |        102 | HELP     |       0 | other  |        2
2015-04-15 01:21:00 |  101121 |        102 | WINE     |       0 | wine   |        2
2015-04-15 01:22:00 |  101121 |        102 | CHECKOUT |      23 | buy    |        2
(10 rows)

4. We may want to use a window function instead of an aggregate. Currently, only aggregates are supported in the core path function in the parameter 'aggregate_func'. However, you can write window functions on the output tuples to achieve the desired result.   Continuing the previous example, let’s say we want to compute average revenue for checkouts within 4 pages of entering the shopping site via the landing page:
SELECT DATE(event_timestamp), user_id, session_id, revenue,
avg(revenue) OVER (PARTITION BY DATE(event_timestamp)) as avg_checkout_rev
FROM path_output_tuples
WHERE page='CHECKOUT'
ORDER BY user_id, session_id;

Result:
    date    | user_id | session_id | revenue | avg_checkout_rev
------------+---------+------------+---------+------------------
2015-04-15 |  100821 |        100 |      39 | 25.6666666666667
2015-04-15 |  101121 |        102 |      15 | 25.6666666666667
2015-04-15 |  101121 |        102 |      23 | 25.6666666666667
(3 rows)

Here we are partitioning the window function by day because we want daily averages, although our sample data set only has a single day.
5. Now we want to do a golden path analysis to find the most successful shopper paths through the site. Since our data set is small, we decide this means the most frequently viewed page just before a checkout is made:
DROP TABLE IF EXISTS path_output, path_output_tuples;
'eventlog',                -- Name of input table
'path_output',             -- Table name to store path results
'session_id',              -- Partition input table by session
'event_timestamp ASC',     -- Order partitions in input table by time
'land:=page=''LANDING'',
wine:=page=''WINE'',
beer:=page=''BEER'',
other:=page<>''LANDING'' AND page<>''WINE'' AND page<>''BEER'' AND  page<>''CHECKOUT''',    -- Symbols for  page types
'array_agg(page ORDER BY session_id ASC, event_timestamp ASC) as page_path',    -- Build array with shopper paths
FALSE                       -- Don't persist matches
);

Now count the common paths and print the most frequent:
SELECT count(*), page_path from
(SELECT * FROM path_output) q
GROUP BY page_path
ORDER BY count(*) DESC
LIMIT 10;

Result:
 count |    page_path
-------+-----------------
5 | {WINE,CHECKOUT}
1 | {BEER,CHECKOUT}
(2 rows)

There are only 2 different paths. The wine page is viewed more frequently than the beer page just before checkout.
6. To demonstrate the use of 'overlapping_patterns', consider a pattern with at least one page followed by and ending with a checkout:
DROP TABLE IF EXISTS path_output, path_output_tuples;
'eventlog',                    -- Name of the table
'path_output',                 -- Table name to store the path results
'session_id',                  -- Partition by session
'event_timestamp ASC',         -- Order partitions in input table by time
$$nobuy:=page<>'CHECKOUT', buy:=page='CHECKOUT'$$,  -- Definition of symbols used in the pattern definition
'(nobuy)+(buy)',         -- At least one page followed by and ending with a CHECKOUT.
'array_agg(page ORDER BY session_id ASC, event_timestamp ASC) as page_path',
FALSE,                        -- Don't persist matches
TRUE                          -- Turn on overlapping patterns
);
SELECT * FROM path_output ORDER BY session_id, match_id;

Result with overlap turned on:
 session_id | match_id |             page_path
------------+----------+-----------------------------------
100 |        1 | {LANDING,WINE,CHECKOUT}
100 |        2 | {WINE,CHECKOUT}
102 |        1 | {LANDING,WINE,CHECKOUT}
102 |        2 | {WINE,CHECKOUT}
102 |        3 | {LANDING,HELP,WINE,CHECKOUT}
102 |        4 | {HELP,WINE,CHECKOUT}
102 |        5 | {WINE,CHECKOUT}
103 |        1 | {LANDING,WINE,HELP,WINE,CHECKOUT}
103 |        2 | {WINE,HELP,WINE,CHECKOUT}
103 |        3 | {HELP,WINE,CHECKOUT}
103 |        4 | {WINE,CHECKOUT}
104 |        1 | {BEER,CHECKOUT}
108 |        1 | {BEER,WINE,CHECKOUT}
108 |        2 | {WINE,CHECKOUT}
(14 rows)

With overlap turned off, the result would be:
 session_id | match_id |             page_path
------------+----------+-----------------------------------
100 |        1 | {LANDING,WINE,CHECKOUT}
102 |        1 | {LANDING,WINE,CHECKOUT}
102 |        2 | {LANDING,HELP,WINE,CHECKOUT}
103 |        1 | {LANDING,WINE,HELP,WINE,CHECKOUT}
104 |        1 | {BEER,CHECKOUT}
108 |        1 | {BEER,WINE,CHECKOUT}
(6 rows)


Note
Please note some current limitations of the path algorithm.
• Window functions cannot currently be used in the parameter 'aggregate_func'. Instead, output the pattern matches and write a SQL query with a window function over the output tuples.
• A given row can only match one symbol. If a row matches multiple symbols, the symbol that comes first in the symbol definition list will take precedence.
• Maximum number of symbols that can be defined is 35.
• The columns 'match_id' and 'symbol' are generated by the path algorithm. If coincidently you have columns in your input data named 'match_id' or 'symbol', the system generated column names will be changed to "__madlib_path_match_id__" and "__madlib_path_symbol__"

Nomenclature

Partition

• scope of rows to be searched for pattern match
• typical examples: user id, session id, portfolio id

Order

• sort order of input rows in partition
• typical example: time

Symbol

• a row of a particular type that you’re searching for, that you want to include in a pattern

Pattern

• regular PostgreSQL pattern match expression of symbols and operators that you want to match across rows

Pattern match

• rows that result from a pattern match expression of symbols
• can be multiple matches per partition

Literature

NOTE: The following links refer to documentation resources for the current PostgreSQL database version. Depending upon your database platform version, you may need to change "current" references in the links 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] PostgreSQL basic statements/assignment operator, http://www.postgresql.org/docs/current/static/plpgsql-statements.html

[2] PostgreSQL pattern matching, http://www.postgresql.org/docs/current/static/functions-matching.html

[3] PostgreSQL aggregate functions, http://www.postgresql.org/docs/current/static/tutorial-agg.html

[4] PostgreSQL window functions, http://www.postgresql.org/docs/current/static/tutorial-window.html