SQL/MP Programming Manual for C

Memory Considerations
HP NonStop SQL/MP Programming Manual for C—429847-008
B-4
Avoiding Memory Stack Overflows
The system allocates real memory in 16 KB pages. If an SQL statement uses only part
of a page, the system allocates the entire page. Therefore, the real memory used by
embedded SQL statements can be larger than the figures shown in Table B-2 on
page B-3.
A program can encounter memory problems in these situations:

The program contains a large number of embedded SQL statements.

The program runs on a system with limited memory (for example, 16 MB or less).

The program runs in a CPU that is also running a large number of other programs.
To reduce the memory use in the extended data segment, follow these guidelines:

Declare only the host variables that your program actually requires.

Declare all host variables in one Declare Section, if possible. The system allocates
the host variables contiguously in one or more pages, rather than allocating each
host variable in a separate page.

Run SQL statements in listing order as often as possible. Thus, the SQL
statements can share many of the pages in the extended data segment.

As a last measure, use dynamic SQL statements. Using dynamic SQL statements
can reduce memory use; however, it can also degrade a program’s performance
because of the additional SQL run-time compilations.
Avoiding Memory Stack Overflows
To avoid memory stack overflows for most SQL statements, the SQL executor needs at
least 3000 words of available stack space. To calculate the approximate stack space
that should be available to run an SQL statement, use this formula:
Stack Space (words) = 3000 +
300 * ( number of referenced tables - 3 )
For example, using this formula, an SQL statement that refers to five tables needs
approximately 3600 words of stack space:
Stack Space (words) = 3000 + ( 300 * (5-3) )
= 3600
The SQL executor handles a stack overflow caused by an SQL statement as follows:

Less than 1024 words of stack space are available. (The limit of 1024 words is an
arbitrary number that is used to prevent problems for existing applications, and
might be increased in a future RVU.)
If there is enough stack space available to call an error handling routine, the SQL
executor returns SQL error 8003. If there is not enough stack space to call the
routine, the executor abends without returning a message.

At least 1024 words of stack space are available.