Re: Searchable chess positions in a Postgress DB

On Wed, Apr 11, 2012 at 09:15:59AM +0200, Sidney Cadot wrote:
> Dear all,
>
> As a hobby project, I am toying around with a database containing
> about 5 million chess games. On average, these games have about 80
> positions (~ 40 moves by both black and white), which means there are
> about 400 million chess positions in there.

Sounds very interesting!

> I am toying around with the ugly idea to make a "Positions" table that
> has a single field for each of the squares, e.g.
>
> CREATE TABLE Position2 (
> GameIndex INTEGER NOT NULL,
> PlyIndex INTEGER NOT NULL,
> a1 "char" NOT NULL,
> a2 "char" NOT NULL,
> -- (60 fields defs omitted)
> h7 "char" NOT NULL,
> h8 "char" NOT NULL
> );
>
> This would allow the creation of indices on each of the 64 fields
> separately, which should help to achieve near-instantaneous position
> query performance, especially after gathering proper statistics for
> all the field-specific indices.
>
> I realize that this design is quite ugly, so I would be interested to
> hear if there are nicer alternatives that can perform equally well.

You could instead create 64 partial indexes on the specific position:

CREATE INDEX i_a1 ON Positions (substring(PseudoFenBoard FROM 1 FOR 1))
WHERE substring(PseudoFenBoard FROM 1 FOR 1) != ' ';

CREATE INDEX i_a2 ON Positions (substring(PseudoFenBoard FROM 2 FOR 1))
WHERE substring(PseudoFenBoard FROM 2 FOR 1) != ' ';

...

so that (a) you don't have to split PseudoFenBoard in more than 1
column, and (b) the indexes are smaller, because they don't index rows
having that position empty (I am assuming that you never ask for games
where a certain position is empty; if you do, then you need to remove
the WHERE clause).

> Also, above I use the 1-byte "char" type. Is this the only type in
> PostGres that is guaranteed to be just a single byte, or are there
> better alternatives? A 13-state enum would be best (listing the 6
> white pieces, 6 black pieces, and 'empty' states for every square on
> the board) but as I understand from the documentation, enums always up
> take 4 bytes per entry.

I think using the 1-byte char is a fairly good choice; you could pack
up your structure in a smaller bit string, but then you add complexity
elsewhere and it might be desirable to keep things simple for now.

Regards,
Dr. Gianni Ciolli - 2ndQuadrant Italia
PostgreSQL Training, Services and Support
gianni(dot)ciolli(at)2ndquadrant(dot)it | www.2ndquadrant.it