[sqlite] What does this code do?

[Ludvig Strigeus]

In sqlite3VdbeRecordCompare()

    /* Read the serial types for the next element in each key. */
    idx1 += sqlite3GetVarint32(&aKey1[idx1], &serial_type1);
    if( d1>=nKey1 && sqlite3VdbeSerialTypeLen(serial_type1)>0 ) break;
    idx2 += sqlite3GetVarint32(&aKey2[idx2], &serial_type2);
    if( d2>=nKey2 && sqlite3VdbeSerialTypeLen(serial_type2)>0 ) break;

What is the purpose of the sqlite3VdbeSerialTypeLen(serial_type1)>0 here?

/Ludvig