Updated DDL for SQL backend

[Derek Atkins]

Quoting Phil Longstaff <plongstaff at rogers.com>:

>> Nah...  Think of a KVP slot like a filesystem directory tree.  Each node
>> can either be a File or a Directory.  The Slot Path walks down the
>> tree (through KVP_FRAMEs) until you get down to a "file" node, a KVP_VALUE
>> of some data-type other that KVP_FRAME.  Just because you don't have
>> a KVP_VALUE of type "list" doesn't mean you can't (or shouldn't) have
>> KVP_VALUE of type KVP_FRAME.
>
> Yes, but without lists, the analogy is each node being either a file or
> a directory which can contain at most 1 file or directory in it.  I
> question the usefulness of all of the directory levels.

You're taking the "no lists" too literally.  A KVP_VALUE of type LIST
is effectively a File with Multiple data contents.  It's NOT a directory.
A Directory (KVP Value of type KVP_VALUE_FRAME) still contains a set of
values!

>> However...  You CAN model it as a flat table!
>> Take a look at the existing PG Backend KVP implementation.   You can
>> model it as:
>>
>>    text(32) object_id NOT NULL,
>>    text slot_path NOT NULL,
>>    enum(kvp_value_types) value_type,
>>    ... values
>>
>> You can construct (and deconstruct) a full kvp tree this way.  No
>> need to reference parents or anything like that.  The slot_path would
>> be a string like "/A/B/C/D".  Each "/" implies a KVP_FRAME with "/"
>> denoting the "root node".
>
> Fine, but without lists, isn't this equivalent to adding '/' to the list
> of chars which can be put into a KVP node name?  Without lists, "/A/B/C"
> and "/A/B/D" are separate independent nodes.  With lists, C and D are
> children in the list "/A/B".

Yes and no..  You can iterate over all members of a KVP_FRAME.   BUT
this iteration is always done in RAM.  The DB only needs to treat a
full set of slots as an atomic unit.

So, yes, you, as the DB Backend, can think of it this way because you're
always loading and storing the full data-set atomically.   In reality
it's not the case, but you can ignore reality for your purposes.

>> The only question with this approach is how you model an empty KVP_FRAME
>> as a leaf-node in the tree.
>
> Yes, this works.  By adding 'empty' to the KVP type enum (at least in
> the DB), you can model empty KVP frames.

Okay...

> Phil

-derek

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord at MIT.EDU                        PGP key available