- ScanTable:
- Read tuples from
a relation stored on disk
- When you use UNIX system calls,
you rely on
the UNIX O.S. to perform the
buffering for you....
Things will be a little different
- Read tuples from
a relation stored on disk
- The header file:
class ScanTable { public: FILE *fp; int fd; char TableName[33]; // Name of the relation DataDescription DataDes[10]; /* Holds data descriptions for upto 10 fields */ int n_fields; /* Actual # fields */ int record_size; char buf[1000]; // Buffer to store a tuple int Open(char *relname); int GetNext(); void Close(); int getAttrType( char *attrName ); // Get data type of attr attrName int getAttrSize( char *attrName ); // Get size of attr attrName void *getAttrPtr( char *attrName ); // Get pointer to attrName in buf[1000] void PrintTuple(); void PrintRelationInfo(); };
- The implementation of
the Open() method:
int ScanTable::Open(char *relname) { /* ------------------------------------------------ Open the catalog to file the relation definition ------------------------------------------------ */ if ( (fp = fopen("catalog", "r")) == NULL ) { cout << "Error: cannot open database catalog `./catalog'" << endl; return 0; } n_fields = 0; record_size = 0; /* ================================================== Read attribute information on relation "relname" ================================================== */ while ( fscanf(fp, "%s %s %c %d %d", DataDes[n_fields].relname, DataDes[n_fields].fieldname, &DataDes[n_fields].fieldtype, &DataDes[n_fields].startpos, &DataDes[n_fields].fieldsize) > 0 ) { if ( strcmp( DataDes[n_fields].relname, relname ) == 0 ) { /* ========================================= This field belongs to the relation ! Adjust the computation of the record size ========================================= */ if ( DataDes[n_fields].startpos + DataDes[n_fields].fieldsize > record_size ) { record_size = DataDes[n_fields].startpos + DataDes[n_fields].fieldsize; } n_fields++; // Accept this field } } fclose(fp); // Close the catalog /* ----------------------------------------- Check if the relation "relname" was found ----------------------------------------- */ if ( n_fields > 0 ) { /* ======================================= Found: open the data file ======================================= */ if ( (fd = open(relname, O_RDONLY)) == -1 ) { cout << "Error: canot open relation file " << relname << endl; return 0; } strcpy( TableName, relname); return 1; // Return success code } else { return 0; // Return error code } }Note:
- I was lazy to allocate
a buffer dynamically.
- Better code is:
char *buf; buf = malloc( record_size + 1 );
- I was lazy to allocate
a buffer dynamically.
- The implementation of the
getNext() method:
int ScanTable::GetNext() { int n; n = read(fd, buf, record_size+1); /* ---------------------------------------- Return a success or error code ---------------------------------------- */ if ( n == record_size+1 ) { return 1; } else { return 0; } }
- The implementation of
the Close() method:
void ScanTable::Close() { close(fd); // Close relation file }
- Here is a simple program on
how to use an
iterator:
ScanTable *f = new ScanTable(); if ( f->Open( "relationName" ) == 0 ) { cout << "Relation not found !" << endl; exit(1); } while ( f->GetNext() != 0 ) { /* ------------------ Print it... ------------------ */ f->PrintTuple(); // I added a print tuple method in the class.... } f->Close();
- Example Program:
(Demo above code)
- Prog files: see /home/cs554000/Handouts/Data
How to run the program:
- To compile: make
- To run: print-data test
- Fact:
- In a programming language,
you must use the
correct varuable type
to store the
corresponing value
Example:
int x; // to store integer value double y; // To store floating point value
- In a programming language,
you must use the
correct varuable type
to store the
corresponing value
- Observe that:
- In ScanTable,
we have used a
char array to
store
all kinds of
values:
char buf[1000]; // Buffer to store a tuple
- In ScanTable,
we have used a
char array to
store
all kinds of
values:
- To access the
values stored in
char buf[ ]
correctly, we
do the following:
- Depending on the
data type of the
attribute value:
- We use the corresponding type of pointer variable to point to the location of the value (stored in char buf[ ])
- We use this pointer to access the value (so it will be using the correct data type !!!)
- Depending on the
data type of the
attribute value:
- The PrintTuple( )
function:
/* ================================================================ PrintTuple(): print the tuple in buf[ ] ================================================================ */ void ScanTable::PrintTuple() { int j; for (j = 0; j < n_fields; j++) { int *i_ptr; // To access an int variable double *f_ptr; // To access a "float" (= double) variable char *c_ptr; // To access a char[ ] variable (string) if ( DataDes[j].fieldtype == 'I' ) { i_ptr = (int *) &(buf[ DataDes[j].startpos]) ; printf("%d ", *i_ptr ); // *i_ptr is an int variable } else if ( DataDes[j].fieldtype == 'F' ) { f_ptr = (double *) &(buf[ DataDes[j].startpos]) ; printf("%lf ", *f_ptr ); // *f_ptr is a double variable } else // String { c_ptr = (char *) &(buf[ DataDes[j].startpos]) ; printf("`%s' ", c_ptr ); // String is a (char *) } } /* ===================================================== The last field in the record is a "tomb stone" flag "Y" = valid record "N" = deleted record ===================================================== */ printf("|| -- Valid flag: %c\n", buf[record_size] ); }