[PATCH][RFC] Make Samba faster by removing o(n) search in ltdb_is_indexed

[Stefan Metzmacher]

Hi Andrew,

> This patch appears to address a performance issue during search
> and modify attempting to determine if an attribute is in an index.
> 
> Our ldb_schema_attribute_by_name() in Samba uses a binary search
> on the schema, and we now set the index flag there in that case.
> 
> Otherwise we fall back to a scan over the index list (no attempt is
> made to try and merge the index values and attribute values in the
> general case). 
> 
> Douglas will do some perf tests on this soon.

Can you please split the patch
- First split out ltdb_index_load() without changing the logic
- Use !ltdb->cache->attribute_indexes instead of
  ltdb->cache->indexlist->num_elements == 0
- change the arguments to ltdb_is_indexed()
- implement the new LDB_ATTR_FLAG_INDEXED related logic
- change samba to use it

While the argument change to ltdb_is_indexed()
will work, I'd actually try to avoid passing
const struct ldb_message *index_list to
ltdb_is_indexed and it's callers.

We should not pass ltdb->cache->indexlist,
I'm wondering if we could just pass 'module' down
and get
struct ltdb_private *ltdb =
talloc_get_type(ldb_module_get_private(module), struct ltdb_private);
and
struct ldb_context *ldb = ldb_module_get_ctx(module);
in ltdb_is_indexed().

Or if it's faster we pass 'ldb' and 'ltdb' to ltdb_is_indexed()
and let the caller get it out of 'module', because some caller
seem to also need them.

I'm also wondering if the optimized code will always
generate the same result as the other one.
What if samba starts to pass LDB_ATTR_FLAG_INDEXED for an
attribute that was not indexed before?
Are there other potential problems, where things can get out of sync?

At the end please let me push it to master, in order to make a separate
"ldb: version ..." commit.

Thanks!
metze

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