Monday, August 17, 2009

Current time? Really?!

"Let S be an <SQL procedure statement> that is not generally contained in a <triggered action>. All <datetime
value function>>s that are contained in <value expression>s that are generally contained, without an intervening
<routine invocation> whose subject routines do not include an SQL function, either in S without
an intervening <SQL procedure statement> or in an <SQL procedure statement> contained in the <triggered
action> of a trigger activated as a consequence of executing S, are effectively evaluated simultaneously.
The time of evaluation of a <datetime value function> during the execution of S and its activated triggers
is implementation-dependent."

The above is a citation of SQL Standard 2008/2003/1999 (part 2 - SQL/Foundation). It's a little nightmare to read, follow and understand, but it's the cause of one behavior generally considered an annoyance in Informix: Inside a stored procedure, all CURRENT references will show the same value.

Let's check the Informix SQL Syntax guide to see how the above ANSI rule is explained in the fine manual:

"SQL is not a procedural language, and CURRENT might not execute in the lexical order of its position in a statement.
You should not use CURRENT to mark the start, the end, nor a specific point in the execution of an SQL statement.

If you use the CURRENT operator in more than once in a single statement,
identical values might be returned by each instance of CURRENT. You cannot rely
on CURRENT to return distinct values each time it executes.

The returned value is based on the system clock and is fixed when the SQL
statement that specifies CURRENT starts execution. For example, any call to
CURRENT from inside the SPL function that an EXECUTE FUNCTION (or
EXECUTE PROCEDURE) statement invokes returns the value of the system clock
when the SPL function starts."

This looks like a more friendly explanation. But it doesn't point out the reason why this is implemented as is, and that reason is mainly for ANSI compliance. In practice, what I usually hear from customers is that this is inconvenient. Many times they are trying to use CURRENT YEAR TO SECOND/FRACTION to find out the time spent on a procedure or parts of it. And it simply doesn't work, because all the values they get are the same. Is there a solution? Yes, if you just need precision up to second:

DBINFO('utc_to_datetime', sh_curtime)

The sh_curtime field of sysmaster:sysshmvals contains the current unix time (number of seconds since January 1 1970). The DBINFO function with the 'utc_to_datetime' converts it into a DATETIME YEAR TO SECOND value.

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