Problem index

Question

Hi
I have a question about index.I want to simulate order by(concept for sql base database) in berkeley db.for examle i want to simulate table like this:
table std_info:
fields:
name,famili,address,std_id,...
and I want to simulate quer like this:
select name from std_info where famili='famili1' order by address
I know how can I define index.
can you give me an example that can be simulate this situations?
thanks alot
regards
saeed

Answer 1

Hi Saeed,
I want to simulate (like statement in sql)
for example I want to simulate query like this:
select * from std_info where address like (%City%);
I know that I must use flag set_range .
but in this situation City is not located in first
part of index so how can I use set_range?If you keep the address as a single field you will need to create a secondary database as an index on address. With a cursor you'll have to iterate and search sequentially each key of this secondary for the pattern %City%. This is somehow similar to the process that hapens in a relational database when you do a SELECT with a WHERE clause containing a LIKE condition (a pattern matching). If the city name is found at a fix location within the address part of the data for every record, than you could construct your secondary only on the city, but this is very unlikely, since the format of an address varies a lot.
A more simpler and elegant approach is to split the address in comprising pieces: street, city, state, zipcode (note that this address format is specific for US, but you can easily adapt to the format that you want).
Thus, the structure that corresponds to the records in "std_info" will look like this:
typedef struct student {
    int std_id;                               /* Student ID */
    char name[MAXFIELD];             /* Student name (first name) */
    char family[MAXFIELD];           /* Student family name (last name) */
     /* Student address, splitted (specific for US addresses */
     char street[MAXFIELD];
     char city[MAXFIELD];
     char state[3];
     char zipcode[6];
} STUDENT;Than, you'll have to create the secondary key generator for the secondary index on city:
/* Secondary key creator for the secondary index on city */
int get_city(DB sdbpCity, const DBT pkey, const DBT pdata, DBT skey)
     char *p = NULL;
     /* point on "std_id" */
     p = (char*) pdata->data;     
     /* move the pointer on "name" */
     p+= sizeof(int);               
     /* move the pointer on "family" */
     p+= (u_int32_t) strlen(p) + 1;
     /* move the pointer on "street" */
     p+= (u_int32_t) strlen(p) + 1;
     /* move the pointer on "city" */
     p+= (u_int32_t) strlen(p) + 1;
     memset(skey, 0, sizeof(DBT));
     skey->data = (void*) p;
     skey->size = (u_int32_t) strlen(p) + 1;
     return(0);
}associate it with the primary db before loading the data, and the querying function may be something similar to this one:
/* Simulate Query 2 */
int query2(DB dbp, DB sdbpCity, const char *searchCity)
     DBC *crs;
     DBT key, data;
     int ret;
     char *p;
     if((ret = sdbpCity->cursor(sdbpCity, NULL, &crs, 0)) != 0){
          sdbpCity->err(sdbpCity, ret, "DB->cursor on secondary failed.");
        return (ret);
     memset(&key, 0, sizeof(DBT));
     memset(&data, 0, sizeof(DBT));
     key.data = (void*) searchCity;
     key.size = (u_int32_t) strlen(searchCity) + 1;
     /* get the first ordered record in the secondary */
     ret = crs->c_get(crs, &key, &data, DB_SET);
     /* retrieve the next ordered records testing for the family name */
     while((ret != DB_NOTFOUND)){
               p = (char*) data.data;
               printf("\nStudent id: %d", (int) *p);
               p += sizeof(int);
               printf("\nName: %s", p);
               p += (u_int32_t) strlen(p) + 1;
               printf("\nFamily: %s", p);
               p += (u_int32_t) strlen(p) + 1;
               printf("\nAddress: %s\n", p); p += (u_int32_t) strlen(p) + 1;
               printf("         %s, ", p); p += (u_int32_t) strlen(p) + 1;
               printf("%s\n", p); p += (u_int32_t) strlen(p) + 1;
               printf("           %s\n\n", p);     
               ret = crs->c_get(crs, &key, &data, DB_NEXT_DUP);
     if(crs != NULL)
          crs->c_close(crs);
     return(0);
}With regard to the suggestions in my previous update here are the according modifications for the secondary composite key creator and the querying function:
/* Secondary key creator for the secondary index on family+address */
int get_family_address(DB sdbp, const DBT pkey, const DBT pdata, DBT skey)
     char p, pFamily, pStreet, pCity, pState, pZipcode;
     u_int32_t dim;
     char *tempbuf;
     p = pFamily = pStreet = pCity = pState = pZipcode = NULL;
     /* point on "std_id" */
     p = (char*) pdata->data;     
     /* move the pointer on "name" */
     p+= sizeof(int);               
     /* move the pointer on "family", retain the address in <pFamily> */
     pFamily = p+= (u_int32_t) strlen(p) + 1;
     /* move the pointer on "street", retain the address in <pStreet> */
     pStreet = p+= (u_int32_t) strlen(p) + 1;
     /* move the pointer on "city", retain the address in <pCity> */
     pCity = p+= (u_int32_t) strlen(p) + 1;
     /* move the pointer on "state", retain the address in <pState> */
     pState = p+= (u_int32_t) strlen(p) + 1;
     /* move the pointer on "Zipcode", retain the address in <pZipcode> */
     pZipcode = p+= (u_int32_t) strlen(p) + 1;
     /* determine the size of the buffer that will store the composite key
      * and allocate space for this buffer */
     dim = (u_int32_t) strlen(pFamily) + strlen(pStreet) +
          strlen(pCity) + strlen(pState) + strlen(pZipcode) + 1;
     tempbuf = (char*) malloc(dim);
     /* set the DB_DBT_APPMALLOC flag in the secondary key DBT */
     memset(skey, 0, sizeof(DBT));
     skey->flags = DB_DBT_APPMALLOC;
     strcpy(tempbuf, pFamily); strcat(tempbuf, pStreet); strcat(tempbuf, pCity);
     strcat(tempbuf, pState); strcat(tempbuf, pZipcode);
    skey->data = (void*) tempbuf;
    skey->size = dim;
    return (0);
/* Simulate Query 1 */
int query1(DB dbp, DB sdbp, const char *searchFamily)
     DBC *crs;
     DBT key, data;
     int ret, walk = 1;
     size_t searchSize;
     char *p;
     searchSize = strlen(searchFamily);
     if((ret = sdbp->cursor(sdbp, NULL, &crs, 0)) != 0){
          sdbp->err(sdbp, ret, "DB->cursor on secondary failed.");
        return (ret);
     memset(&key, 0, sizeof(DBT));
     memset(&data, 0, sizeof(DBT));
     key.data = (void*) searchFamily;
     key.size = (u_int32_t) strlen(searchFamily) + 1;
     /* get the first ordered record in the secondary */
     ret = crs->c_get(crs, &key, &data, DB_SET_RANGE);
     /* retrieve the next ordered records testing for the family name */
     while((ret != DB_NOTFOUND) && walk){
          if(strncmp((const char*)key.data, searchFamily, searchSize))
               walk = 0;
          else {
               p = (char*) data.data;
               printf("\nStudent id: %d", (int) *p);
               p += sizeof(int);
               printf("\nName: %s", p);
               p += (u_int32_t) strlen(p) + 1;
               printf("\nFamily: %s", p);
               p += (u_int32_t) strlen(p) + 1;
               printf("\nAddress: %s\n", p); p += (u_int32_t) strlen(p) + 1;
               printf("         %s, ", p); p += (u_int32_t) strlen(p) + 1;
               printf("%s\n", p); p += (u_int32_t) strlen(p) + 1;
               printf("           %s\n\n", p);     
          ret = crs->c_get(crs, &key, &data, DB_NEXT);
     if(crs != NULL)
          crs->c_close(crs);
     return(0);
}Also, you could set a comparison function on the primary db for sorting correctly the integer keys, and you could set this function as a duplicate comparison function for the records in the secondary databases, using the "DB->set_bt_compare" and, respectively, the "DB->set_dup_compare" methods:
/* Btree comparison function for primary db keys
* also used when sorting duplicates in secondary dbs */
int comp_fcn(DB dbp, const DBT dbt1, const DBT *dbt2)
     int ai, bi;
     memcpy(&ai, dbt1->data, sizeof(int));
     memcpy(&bi, dbt2->data, sizeof(int));
     return (ai - bi);
}Regards,
Andrei