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Oracle® Fusion Middleware CQL Language Reference for Oracle Event Processing
11g Release 1 (11.1.1.7)

Part Number E12048-09
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3 Pseudocolumns

This chapter provides a reference for Oracle Continuous Query Language (Oracle CQL) pseudocolumns, which you can query for but which are not part of the data from which an event was created.

This chapter includes the following sections:

3.1 Introduction to Pseudocolumns

You can select from pseudocolumns, but you cannot modify their values. A pseudocolumn is also similar to a function without arguments (see Section 1.1.11, "Functions").

Oracle CQL supports the following pseudocolumns:

3.2 ELEMENT_TIME Pseudocolumn

Every stream element of a base stream or derived stream (a view that evaluates to a stream) has an associated element time.The ELEMENT_TIME pseudo column returns this time as an Oracle CQL native type bigint.

Note:

ELEMENT_TIME is not supported on members of an Oracle CQL relation. For more information, see Section 1.1.1, "Streams and Relations".

This section describes:

For more information, see:

3.2.1 Understanding the Value of the ELEMENT_TIME Pseudocolumn

The value of the ELEMENT_TIME pseudocolumn depends on whether or not you configure the stream element's channel as system- or application-timestamped.

3.2.1.1 ELEMENT_TIME for a System-Timestamped Stream

In this case, the element time for a stream element is assigned by the Oracle Event Processing system in such a way that subtracting two values of system-assigned time will give a duration that roughly matches the elapsed wall clock time.

For more information, see "System-Timestamped Channels" in the Oracle Fusion Middleware Developer's Guide for Oracle Event Processing for Eclipse.

3.2.1.2 ELEMENT_TIME for an Application-Timestamped Stream

In this case, the associated element time is assigned by the application using the application assembly file wlevs:expression element to specify a derived timestamp expression.

Oracle Event Processing processes the result of this expression as follows:

For more information, see "Application-Timestamped Channels" in the Oracle Fusion Middleware Developer's Guide for Oracle Event Processing for Eclipse.

3.2.1.2.1 Dervied Timestamp Expression Evalutes to int or bigint

If the dervied timestamp expression evaluates to an Oracle CQL native type of int, then it is cast to and returned as a corresponding bigint value. If the expression evaluates to an Oracle CQL native type of bigint, that value is returned as is.

3.2.1.2.2 Dervied Timestamp Expression Evalutes to timestamp

If the derived timestamp expression evaluates to an Oracle CQL native type of timestamp, it is converted to a long value by expressing this time value as the number of milliseconds since the standard base time known as "the epoch", namely January 1, 1970, 00:00:00 GMT.

3.2.2 Using the ELEMENT_TIME Pseudocolumn in Oracle CQL Queries

This section describes how to use ELEMENT_TIME in various queries, including:

3.2.2.1 Using ELEMENT_TIME With SELECT

Example 3-1 shows how you can use the ELEMENT_TIME pseudocolumn in a select statement. Stream S1 has schema (c1 integer). Given the input stream that Example 3-2 shows, this query returns the results that Example 3-3 shows. Note that the function to_timestamp is used to convert the Long values to timestamp values.

Example 3-1 ELEMENT_TIME Pseudocolumn in a Select Statement

<query id="q4"><![CDATA[ 
    select 
        c1, 
        to_timestamp(element_time) 
    from 
        S1[range 10000000 nanoseconds slide 10000000 nanoseconds]
]]></query>

Example 3-2 Input Stream

Timestamp   Tuple
 8000       80
 9000       90
13000       130
15000       150
23000       230
25000       250

Example 3-3 Output Relation

Timestamp   Tuple Kind  Tuple
 8000       +           80,12/31/1969 17:00:08
 8010       -           80,12/31/1969 17:00:08
 9000       +           90,12/31/1969 17:00:09
 9010       -           90,12/31/1969 17:00:09
13000       +           130,12/31/1969 17:00:13
13010       -           130,12/31/1969 17:00:13
15000       +           150,12/31/1969 17:00:15
15010       -           150,12/31/1969 17:00:15
23000       +           230,12/31/1969 17:00:23
23010       -           230,12/31/1969 17:00:23
25000       +           250,12/31/1969 17:00:25
25010       -           250,12/31/1969 17:00:25

If your query includes a GROUP BY clause, you cannot use the ELEMENT_TIME pseudocolumn in the SELECT statement directly. Instead, use a view as Section 3.2.2.2, "Using ELEMENT_TIME With GROUP BY" describes.

3.2.2.2 Using ELEMENT_TIME With GROUP BY

Consider query Q1 that Example 3-4 shows. You cannot use ELEMENT_TIME in the SELECT statement of the query because of the GROUP BY clause.

Example 3-4 Query With GROUP BY

<query id="Q1"><![CDATA[ 
    SELECT
        R.queryText AS queryText,
        COUNT(*) AS queryCount
    FROM
        queryEventChannel [range 30 seconds] AS R
    GROUP BY
        queryText
]]></query>

Instead, create a view as Example 3-5 shows. The derived stream corresponding to V1 will contain a stream element each time (queryText, queryCount, maxTime) changes for a specific queryText group.

Example 3-5 View

<view id="V1"><![CDATA[ 
    ISTREAM (
        SELECT
            R.queryText AS queryText,
            COUNT(*) AS queryCount,
            MAX(R.ELEMENT_TIME) as maxTime
        FROM
            queryEventChannel [range 30 seconds] AS R
        GROUP BY
            queryText
    )
]]></view>

Note that the element time associated with an output element of view V1 need not be the same as the value of the attribute maxTime for that output event. For example, as the window slides and an element from the queryEventChannel input stream expires from the window, the queryCount for that queryText group would change resulting in an output. However, since there was no new event from the input stream queryEventChannel entering the window, the maxTime among all events in the window has not changed, and the value of the maxTime attribute for this output event would be the same as the value of this attribute in the previous output event.

However, the ELEMENT_TIME of the output event corresponds to the instant where the event has expired from the window, which is different than the latest event from the input stream, making this is an example where ELEMENT_TIME of the output event is different from value of "maxTime" attribute of the output event.

To select the ELEMENT_TIME of the output events of view V1, create a query as Example 3-6 shows.

Example 3-6 Query

<query id="Q1"><![CDATA[ 
    SELECT
        queryText,
        queryCount,
        ELEMENT_TIME as eventTime
    FROM
        V1
]]></query>

3.2.2.3 Using ELEMENT_TIME With PATTERN

Example 3-7 shows how the ELEMENT_TIME pseudocolumn can be used in a pattern query. Here a tuple or event matches correlation variable Nth if the value of Nth.status is >= F.status and if the difference between the Nth.ELEMENT_TIME value of that tuple and the tuple that last matched F is less than the given interval as a java.lang.Math.Bigint(Long).

Example 3-7 ELEMENT_TIME Pseudocolumn in a Pattern

...
PATTERN (F Nth+? L)
        DEFINE
            Nth AS 
                Nth.status >= F.status
                AND 
                Nth.ELEMENT_TIME - F.ELEMENT_TIME < 10000000000L,
            L AS 
                L.status >= F.status 
                AND 
                count(Nth.*) = 3
                AND L.ELEMENT_TIME - F.ELEMENT_TIME < 10000000000L 
...