This function is used to collate data from events that share a key. Often the groupBy() function can be used for this, but if there are too many keys (defaulting to 100,000) then the result is imprecise since some random subset of keys is left out of the result once the limit is reached.

ParameterTypeRequiredDefault ValueDescription
collectarray of stringsoptional[a](none) Specifies columns to include.
field[b]array of stringsrequired  Specifies which field in the event (log line) that must match the given column value.
limitnumberoptional[a]20000 Specifies the maximum number of rows in the subquery
  Minimum1 
  MaximumMAX_STATE_LIMIT 
postfilterbooleanoptional[a]false Re-run the and of the where clauses after collating results. If all fields needed for satisfying the where clauses are provided as values for collect, this will eliminate false positives in the output.
prefilterbooleanoptional[a]false Only pass values matching at least one of the where clauses into the embedded groupBy().
selectarray of stringsoptional[a](none) Specifies columns to include.
where[filter]required  The subquery to execute producing the values to join with.

[a] Optional parameters use their default value unless explicitly set.

[b] The parameter name field can be omitted.

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Say you want to run a query like this:

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groupby(field=email_id, function=[collect(from), collect(to)])
| from=peter AND to=anders

If there are many email_ids then the state size is exhausted. Even if you rewrite it like this to:

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from=peter OR to=anders
| groupby(field=email_id, function=[collect(from), collect(to)])
| from=peter AND to=anders

because either there are many emails from Peter or to Anders.

With selfJoin() you specify a join key (the field argument), and a series of tests (the where clauses); it will then perform the above operation in a two-phase way so that only those log lines with an email_id for which there exists both a from=peter event and a to=anders event are passed into the groupBy(). This is done in a probabilistic fashion, by using a bloom filter.

The embedded groupBy() then uses either collect() or selectLast() on the specified fields depending on the value of one of the select or collect parameter to the selfJoin() function.

The above query can be simplied (and be made more efficient) by using the selfJoin() function:

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selfJoin(field=email_id, where=[{from=peter},{to=anders}])

selfJoin() limits the number of matching join keys to what is configured in GroupMaxLimit dynamic configuration. Default is 20,000. These limits apply to the number of join key values that satisfy the collective requirements of the where clauses. In this case, the function would return the number of emails from Peter to Anders.

Important

selfJoin() is probabilistic and the result can contain false positives.

matches false positive rate number of false positives
1000 0.00000% 0.0
10000 0.00029% 0.0
20000 0.00224% 0.4
25000 0.00434% 1.1
50000 0.03289% 16.4

If, for example, the where clauses (along with any preceding filtering) limit the matching IDs to 25,000 elements, then 1.1 of those will be false positives on average.

Warning

The selfJoin() does two passes over the data and can therefore not run truly live.

selfJoin() Examples

Click + next to an example below for to get the full details.

Filter and Collect Values in Same Table

Retrieves all emails sent from one given person to another given person using the selfJoin() function

Query
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selfJoin(email_id, where=[{from=*peter*}, {to=*anders*}], collect=[from,to])
Introduction

The selfJoin() function is a join query that matches events across the same event sets. selfJoin() joins an event set to itself and allows you to combine events from the same table based on two fields in the same event. In order to do this, the event set must have a common field with a unique ID, a primary field, and a secondary (or subquery) field that will be matched to each other. In this example, emails are logged with one event for each header (each email has its own ID) and the selfJoin() function is used to find and collect all emails sent from one given person to another given person. Notice, that this query does two passes over the data and, therefore, cannot be used in a live query.

Step-by-Step

  1. Starting with the source repository events.

  2. logscale
    selfJoin(email_id, where=[{from=*peter*}, {to=*anders*}], collect=[from,to])

    Finds and collects all the values in the emails_id field that correspond to mails from Peter to Anders.

  3. Event Result set.

Summary and Results

The query is used to find and collect all emails sent from one given person to another person. In general, the selfJoin() function is useful for narrowing down a set of events in a fairly efficient manner, in cases where the total set of events is too voluminous.