3 Pseudocolumns
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 is provided.
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 Functions).
Oracle CQL supports the following pseudocolumns:
3.2 ELEMENT_TIME Pseudocolumn
In CQL, every stream event is associated with a timestamp. The ELEMENT_TIME pseudo column returns the timestamp of the stream event. The datatype of ELEMENT_TIME pseudo column is Oracle CQL native bigint type. The unit of timestamp value returned by ELEMENT_TIME is in nanoseconds.
Note:
ELEMENT_TIME is not supported on members of an Oracle CQL relation. For more information, see Streams and Relations.
This section describes:
For more information, see:
3.2.1 Understanding the Value of the ELEMENT_TIME Pseudocolumn
The value of ELEMENT_TIME for each stream event is the timestamp of that event. The timestamp of stream event depends on the stream definition and source.
3.2.1.1 ELEMENT_TIME for a System-Timestamped Stream
If source stream is a system timestamped stream, then the timestamp for a stream event is assigned by computing System.nanoTime(). For each event, ELEMENT_TIME pseudo column outputs the event's timestamp.
For example, consider a system timestamped stream defined as: tktest_S1(c1 integer).
select ELEMENT_TIME, to_timestamp(ELEMENT_TIME) from tktest_S1
Input (c1) Output (timestamp: element_time, to_timestamp(element_time))
10 12619671878392750:+ 12619671878392750,05/26/1970 18:27:51
20 12619671889193750:+ 12619671889193750,05/26/1970 18:27:51
30 12619671890093750:+ 12619671890093750,05/26/1970 18:27:51
40 12619671891399750:+ 12619671891399750,05/26/1970 18:27:51
50 12619671896472750:+ 12619671896472750,05/26/1970 18:27:51Note:
The output may vary for each execution and also depends on the machine as timestamp is computed by calculatingSystem.nanoTime().
3.2.1.2 ELEMENT_TIME for an Application-Timestamped Stream
If source stream is an application timestamped stream, then timestamp for stream event is assigned by computing the application timestamp expression. The unit of computed timestamp value is always in nanoseconds. ELEMENT_TIME pseudo column outputs the event's timestamp.
For example, consider an application timestamped stream defined as tktest_S1(C1 integer, c2 bigint) and application timestamp expression as: c2*1000000000L.
select ELEMENT_TIME, to_timestamp(ELEMENT_TIME) from tktest_S1
Input(c1,c2) Output(timestamp: element_time, to_timestamp(element_time))
10, 10 10000000000:+ 10000000000,12/31/1969 17:00:10
20, 20 20000000000:+ 20000000000,12/31/1969 17:00:20
30, 30 30000000000:+ 30000000000,12/31/1969 17:00:30
40, 40 40000000000:+ 40000000000,12/31/1969 17:00:40
50, 50 50000000000:+ 50000000000,12/31/1969 17:00:50In the above query, the timestamp of each event is computed by computing c2*1000000000L for each event. You can see that ELEMENT_TIME is same as timestamp of the event.
3.2.1.2.1 Derived Timestamp Expression Evaluates to int or bigint
If the derived 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 Derived Timestamp Expression Evaluates 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.
3.2.1.3 ELEMENT_TIME for a Subquery
If source stream is received from a subquery, then CQL does not support ELEMENT_TIME on the subquery results.
The following example depicts the scenario which is not supported.
SELECT ELEMENT_TIME FROM ( ISTREAM(SELECT * FROM SYSTS_STREAM[RANGE 1 HOUR SLIDE 5 MINUTES])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
The following example shows how you can use the ELEMENT_TIME pseudocolumn in a select statement. Stream S1 has schema (c1 integer). 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 8000 80 9000 90 13000 130 15000 150 23000 230 25000 250
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 Using ELEMENT_TIME With GROUP BY describes.
3.2.2.2 Using ELEMENT_TIME With GROUP BY
You cannot use ELEMENT_TIME in the SELECT statement of the query because of the GROUP BY clause.
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 maxTimeattribute of the output event.
To select the ELEMENT_TIME of the output events of view V1, create a query.
<query id="Q1"><![CDATA[
SELECT queryText, queryCount, ELEMENT_TIME as eventTime FROM V1
]]></query>
3.2.2.3 Using ELEMENT_TIME With PATTERN
The following example 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).
... 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 ...