Re: (ITS#8908) LMDB Documentation for MDB_XXX_MULTIPLE

[hyc@symas.com]

karl@waclawek.net wrote:
> --00000000000018b43a057492d87f
> Content-Type: text/plain; charset="UTF-8"
> 
> As far as point 2 is concerned I was basing it on this piece of code, where
> it looks like MDB_GET_MULTIPLE just checks some flags and then goes to the
> fetchm label where the code does nothing but retrieve data:

You're not reading the code correctly. See below.
> 
> case MDB_GET_MULTIPLE:
> if (data == NULL || !(mc->mc_flags & C_INITIALIZED)) {
> rc = EINVAL;
> break;
> }
> if (!(mc->mc_db->md_flags & MDB_DUPFIXED)) {
> rc = MDB_INCOMPATIBLE;
> break;
> }
> rc = MDB_SUCCESS;
> if (!(mc->mc_xcursor->mx_cursor.mc_flags & C_INITIALIZED) ||
> (mc->mc_xcursor->mx_cursor.mc_flags & C_EOF))
> break;
> goto fetchm;
> case MDB_NEXT_MULTIPLE:
> if (data == NULL) {
> rc = EINVAL;
> break;
> }
> if (!(mc->mc_db->md_flags & MDB_DUPFIXED)) {
> rc = MDB_INCOMPATIBLE;
> break;
> }
> rc = mdb_cursor_next(mc, key, data, MDB_NEXT_DUP);
> if (rc == MDB_SUCCESS) {
> if (mc->mc_xcursor->mx_cursor.mc_flags & C_INITIALIZED) {
> MDB_cursor *mx;
> fetchm:
> mx = &mc->mc_xcursor->mx_cursor;
> data->mv_size = NUMKEYS(mx->mc_pg[mx->mc_top]) *
> mx->mc_db->md_pad;
> data->mv_data = METADATA(mx->mc_pg[mx->mc_top]);
> mx->mc_ki[mx->mc_top] = NUMKEYS(mx->mc_pg[mx->mc_top])-1;

^^^ this line advances the cursor.

> } else {
> rc = MDB_NOTFOUND;
> }
> }
> break;
> 
> 
> On Tue, Aug 28, 2018 at 8:26 PM Howard Chu <openldap-its@openldap.org>
> wrote:
> 
>>> Full_Name: Karl Waclawek
>>> Version: LMDB 0.9.22
>>> OS: Windows 10
>>> URL: ftp://ftp.openldap.org/incoming/
>>> Submission from: (NULL) (2607:fea8:7a80:814:3ca9:e8f6:801c:d6f8)
>>>
>>>
>>> The documentation in lmdb.h for MDB_GET_MULTIPLE, MDB_NEXT_MULTIPLE and
>>> MDB_PREV_MULTIPLE seems to have two issues:
>>>
>>> 1) The documentation indicates that both, the key and data, are
>> returned, but
>> in
>>> reality only the data is returned.
>>
>> Looks like you're right. Since these are DUPs the key would be the same
>> every time so there's no need to return it on each call.
>> Doc fixed in git mdb.master.
>>
>>> 2) The documentation states that MDB_GET_MULTIPLE moves the cursor. This
>> does
>>> not seem to be true.
>>
>> The doc is correct here. The cursor is advanced far enough that a
>> MDB_NEXT_MULTIPLE will return the following data, as opposed to just
>> returning the same data again.
>>
> 



-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/