Re: Searchable chess positions in a Postgress DB

On 11/04/12 19:15, 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.
>
> I have written code to extract these positions, and now I want to put
> them into a Postgres database. Specifically, I want to do this in a
> way that allows *fast* lookups of positions, e.g. "give me all
> positions that have a White King on c4 and either a Black Bishop or
> White Knight on f7".
>
> Currently, my "Positions" table looks like this:
>
> Column | Type | Modifiers
> -------------------+---------+-----------
> gameindex | integer | not null
> plyindex | integer | not null
> pseudofenboard | text | not null
> fenside | text | not null
> fencastling | text | not null
> fenenpassant | text | not null
> possiblemovecount | integer | not null
> isincheck | boolean | not null
> Indexes:
> "positions_pkey" PRIMARY KEY, btree (gameindex, plyindex)
> Foreign-key constraints:
> "positions_gameindex_fkey" FOREIGN KEY (gameindex) REFERENCES
> games(gameindex)
>
> The "PseudoFenBoard" field currently holds a string describing the
> position. For example, the starting position of chess looks like this:
>
> "rnbqkbnr/pppppppp/________/________/________/________/PPPPPPPP/RNBQKBNR"
>
> This design allows me to formulate the kind of positional queries that
> I want (by using regular expression matching), but executing them will
> involve a slow, linear traversal of the 400M table rows, which is not
> desirable.
>
> 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.
>
> 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.
>
> Any ideas for improvement would be greatly appreciated.
>

How aboutsomething like the following (game and postion would have more
fields in practice, like comments and where played)?

DROP TABLE IF EXISTS game CASCADE;

CREATE TABLE game
(
id int PRIMARY KEY,
name_white text,
name_black text,
played timestamptz
);

CREATE TABLE position
(
id int PRIMARY KEY,
game_id int REFERENCES game (id),
ply int
);

CREATE TABLE piece
(
id int PRIMARY KEY,
position_id int REFERENCES position (id),
rank char, -- 1...8 from white's perspective
file char, -- a...h
white boolean,
type char -- P.R,N,B,K,Q
);

CREATE UNIQUE INDEX square ON piece (rank, file, type, white);

SELECT
p.position_id
FROM
piece p
WHERE
( p.white
AND p.type = 'K'
AND p.file = 'c'
AND p.rank = '4'
)
AND
(
((NOT p.white AND p.type = 'B') OR (p.white AND p.type = 'K'))
AND p.file = 'f'
AND p.rank = '7'
);

Cheers,
Gavin