Re: Char & Varchar2

Thomas Ruschak wrote:
>
> Hey, can anyone give me a quick couple of lines overview on when
> to use char & when varchar2? What are the overhead costs associated with
> varchar2?

The VARCHAR datatype is acutally implemented as a structure in C. The data structure has the following form: (note: that the arr field is of type 'unsigned char' -- Nobody at Oracle can give you a good reason for this)

(gdb) p m_cancel
$1 = {
  len = 1,
  arr = "Y\000"
}
(gdb)

Where m_cancel.len carries the length (in bytes) of the string read from the database and the m_cancel.arr carries the actual value of the string. In this case, the VARCHAR variable m_cancel is a 1 byte string with the value of 'Y'.

For the VARCHAR string, to copy data into the string:

        strcpy(string.arr, "string to copy");

To assign the length of the string:

        string.len = strlen(string);

To NULL terminate the string:

        string.len+1 = '\0';

The VARCHAR datatype is quite useful because it has the length assigned on output from the database. This means you don't have to use a call to strlen and you can do useful stuff like this:

[Quoted]         VARCHAR oracle_string[80];
        char    new_string[20];

        strncpy(new_string, oracle_string, oracle_string.len);

Thus, you know the sizeof the oracle_string from its len field.

Jeff

-- 
Senior Software Engineer
Fairway Systems, Inc.

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