Re: Patch for Improved Syntax Error Reporting

Bruce Momjian <pgman@candle.pha.pa.us> writes:
>> significant new effort from the client app is where it reformats queries
>> before sending them on to the parser ... but that is knowledge you've
>> already built into the client, no?  You just have to remember what you
>> did so that you can map a backend error index into the original text.

> That seems particularly hard.

No, it's absolutely and utterly trivial.  As you collapse the query to
what you send, you are copying it from an input buffer to an output
workspace, no?  As you do this, you build an index array that holds the
input index of each output character.  Then when you get an error index
from the backend, you look in this array to find the source-text index
you want to point at.  Then you do what Neil originally proposed to do
in the backend (which was what, about a dozen lines of code?) to produce
an error report, if the style of error report he proposed was what makes
sense for your client app.  Or you do something different if that's what
makes sense for your app.

    ninchars = strlen(inputbuf);
    outbuf = (char *) malloc((ninchars+1) * sizeof(char));
    maparray = (int *) malloc((ninchars+1) * sizeof(int));

    outindex = 0;
    for (inindex = 0 ; inindex < ninchars; inindex++)
    {
        if (want to send this char to backend)
        {
            outbuf[outindex] = inputbuf[inindex];
            maparray[outindex] = inindex;
            outindex++;
        }
    }

    outbuf[outindex] = inputbuf[inindex];   /* terminating \0 */
    maparray[outindex] = inindex;

This is essentially three more lines than what you had anyway
(everything except the lines mentioning maparray was there before).
When you get an error index back from the backend, it takes one
more line of code to map it back to an input-string index.
Not exactly rocket science, IMHO.

Four additional lines of code on the client side seems a *very* small
price to pay for giving client apps the freedom to do the right thing
for their style of user interface.

            regards, tom lane

PS: okay, five lines.  I forgot to count free(maparray) ;-)