Or just stick your own identifier byte at the beginning of each data value. (Or key, if you're trying to use different type'd keys.)Hallvard Breien Furuseth wrote:
On 02. des. 2016 12:26, Tim Uckun wrote:
I am writing a wrapper for liblmdb and one thing I was able to do was to
store simple integers and floats as the value into the database. This seems
to work pretty well but I am wondering if this is somehow against best
practice.
One bad thing about this is that when you are doing a seek or a cursor
operation the code does not know what kind of data is at the record. I have
some routines to cast them back from the pointers but the user has to know
what to cast them to. It would be very handy if the MDB_val struct had a
field for metadata, even if it just a byte.
LMDB handles binary data fine. The problem is ordering:
Use a single datatype for each DB, and mdb_set_compare()
mdb_set_dupsort() to order data in that DB properly. Or
MDB_INTEGERKEY/MDB_INTEGERDUP which do this for unsigned int/size_t.
If that is not feasible and you are mixing different datatypes in a
single database, you need to tag each data item so the compare function
can figure out how to parse and order them all. Note that a slow
compare function can degrade LMDB's performance.
Note that the compare functions may receive unaligned values. Though
you can hack alignment 2, 4, or sizeof(size_t) = 4 or 8 by ensuring
*every* key and value in a DB has size = multiple of that alignment.
Keys always have alignment of at least 2. Hmm, I should add some
code so data will too except when both key and data have uneven size.
Requests to add a user-defined argument to the compare functions have
been rejected. If you need that, maybe your wrapper can emulate it by
setting thread-specific data before calling LMDB, and then your sort
function can read that data. That gets cumbersome, though.
If this is all generic, maybe define a DB with meta-info where your
wrapper can look up which datatypes (sizes, compare functions, whatever)
are in which database.
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/