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 1.1.11, "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::=.

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

Section 3.2.1, "Using ELEMENT_TIME With SELECT"
Section 3.2.2, "Using ELEMENT_TIME With GROUP BY"
Section 3.2.3, "Using ELEMENT_TIME With PATTERN"

3.2.1 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, "Using ELEMENT_TIME With GROUP BY" describes.

3.2.2 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>

To select these query elements, 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>

3.2.3 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 
...