Query may fail due to constant folding and propagation

Details

Detail name Value
Changelog Number 12463
Type Bug
Status Resolved
Affected Versions Exasol 6.2.0, Exasol 7.0.0, Exasol 7.1.0
Fix Versions Exasol 6.2.16, Exasol 7.0.11
Resolution Date 2021-07-15

Background

In very rare cases, the constant folding and propagation algorithm in Exasol can lead to access of invalid memory. This invalid memory access can either cause an internal server error or an incorrect data exception.

Description

Only if the constant folding algorithm affects a query in a certain way this issue can occur. Thus, it is not possible to provide a list with all scenarios. Currently, this is the only known scenario:

  • Use of LAG/LEAD in a common table expression.
  • Use of a constant as argument of LAG/LEAD (e.g., CURRENT_TIMESTAMP).

Preparation:

CREATE OR REPLACE TABLE T1 ("MEASURE_TIME" TIMESTAMP);

Example 

WITH
    utimes AS
    (   SELECT
            LEAD(measure_time, 1, CURRENT_TIMESTAMP) over (ORDER BY measure_time) AS shutdown_time FROM T1
    )
SELECT * FROM utimes;

Workaround

This depends on the query and is difficult to do as it is necessary to influence the constant folding algorithm. Please contact our support if this issue occurs.
For the above scenario, a cast is sufficient:

WITH
    utimes AS
    (   SELECT
            LEAD(measure_time, 1, CAST(CURRENT_TIMESTAMP AS TIMESTAMP)) over (ORDER BY measure_time) AS shutdown_time FROM T1
    )
SELECT * FROM utimes;

Fix

The affected queries work without error.