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Re: Subject: Re: Fast key range iteration

James Rouzier wrote:

Here is a faster to do way to do what you want.
Just get the last key of your range and while iterating compare the data
pointer of the last key to see if you reach the end instead of the it's content.
As long as you are in the same transaction the data pointer of the last key
should never change. (Someone correct me if I am wrong).
Right, that will work in a read transaction. In a write transaction, if any writes are happening while iterating thru, it's possible for the data pointers to change. 

Here is a quick example of what I mean.

   MDB_cursor *cursor;
   MDB_val first_key, key, data;
   MDB_val last_key1, last_key2, last_data;
   int rc;

//Get the last key
   rc = mdb_cursor_get (cursor, &last_key1, &last_data, MDB_SET_RANGE);
    //If the current position is not equal move back one spot
   if (rc == MDB_SUCCESS && mdb_cmp (mdb_cursor_txn (cursor), mdb_cursor_dbi
(cursor), &last_key1, &last_key2)) {
     rc = mdb_cursor_get (cursor, &last_key1, &last_data, MDB_PREV);
   }
   //This means that nothing is equal or greater than the last key or the last
key is at the first position
   if (rc) {
     last_data.mv_data = NULL;
   }
   //Get the first key
   rc = mdb_cursor_get (cursor, &first_key, &data, MDB_SET_RANGE);
   if(rc == MDB_SUCCESS) {
     do {
     } while (mdb_cursor_get (cursor, &key, &data, MDB_NEXT) == MDB_SUCCESS &&
data.mv_data != last_data.mv_data);
   }

 > Hi all!

 > Is there a way to iterate with all key-value pairs in LMDB database, where
all keys is in specified range?
 > I'm trying to use mdb_cursor_get() with MDB_SET_RANGE to search first pair.
 > Then I see only one way - use mdb_cursor_get() with MDB_NEXT and compare
key by memcmp().
 > But it seems absolutely not effective.
 >
 > Is there another solution for my task?
 >
 > Thanks!




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