3 Pseudocolumns

A pseudocolumn behaves like a stream element, but is not actually part of the tuple.

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 1.1.9, "Functions").

Oracle CQL supports the following pseudocolumns:

Section 3.2, "ELEMENT_TIME Pseudocolumn"

3.2 ELEMENT_TIME Pseudocolumn

The ELEMENT_TIME pseudocolumn returns the timestamp value associated with a given stream element as a java.lang.Math.Bigint(Long).

For syntax, see pseudo_column::=.

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

Example 3-4 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 3-4 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 
...