使用nvarchar（max）时文本溢出？

Question

I've run into a strange situation that seems to involve long text overflowing. I'm using fn_pcre_replace (from the xp_pcre extended stored procedure ) on an nvarchar(max) column, and when I replace with more characters than were there originally, it returns NULL if the original string was over 8000 characters long.

For example, this

Select master.dbo.fn_pcre_replace(Overview,'a', 'X')
from ProjectContent 

works as expected, but this

Select master.dbo.fn_pcre_replace(Overview,'a', 'XXX')
from ProjectContent 

returns NULL in most cases (apparently, wherever Len(Overview)>8000 ).

Is this a limitation of xp_pcre , or is there something I can do in my SQL to make this work?

Answer 1

This is a limitation of xp_pcre . Looking at the source:

    switch (bType)
    {
    case SRVBIGCHAR:
    case SRVBIGVARCHAR:
        break;
    default:
        throw XpException(
            StringBuilder()
            << "Invalid data type on parameter "
            << paramNum
            << " (should be CHAR or VARCHAR)."
            );
    }

I can conclude these two values (from <srv.h> ) permit up to a maximum of 8000 characters. SRVBIGVARCHAR is

Variable-length character data type, length 0 to 8000 bytes.

You would need to update the source and recompile with support for SRVTEXT or SRVVARCHAR so it is not a limiting factor when using external procedures.