This chapter includes the following sections:
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:
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
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:
The value of the
ELEMENT_TIME pseudocolumn depends on whether or not you configure the stream element's channel as system- or application-timestamped.
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.
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.
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.
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.
This section describes how to use
ELEMENT_TIME in various queries, including:
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.
<query id="q4"><![CDATA[ select c1, to_timestamp(element_time) from S1[range 10000000 nanoseconds slide 10000000 nanoseconds] ]]></query>
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 220.127.116.11, "Using ELEMENT_TIME With GROUP BY" describes.
Q1 that Example 3-4 shows. You cannot use
ELEMENT_TIME in the
SELECT statement of the query because of the
GROUP BY clause.
<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 (
maxTime) changes for a specific
<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 3-7 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
>= 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