For starters gid should probably be a numeric type. integer should be good enough or bigint if the key space shouldn't be big enough. Much smaller footprint, faster processing than with character data, faster and smaller indexes.
More importantly, to improve performance I suggest database normalization.
Quote:
There is a fairly regular pattern where each word appears about 1000 times.
Create a separate table for unique words:
CREATE TABLE word (
word_id serial
, word text
);
Fill it with unique instances of word in your big_tbl:
INSERT INTO word (word)
SELECT DISTINCT word
FROM big_tbl
ORDER BY word;
ORDER BY is optional, not needed for query at hand. But it speeds up index creation and might be cheaper overall.
The table should be small in comparison: only ~ 50k rows for 50M rows in your big table.
Add indexes after filling the table:
ALTER TABLE word
ADD CONSTRAINT word_word_uni UNIQUE (word) -- essential
, ADD CONSTRAINT word_word_id_pkey PRIMARY KEY (word_id); -- expendable?
If those are read-only tables, you can do without the pk. It's not relevant to the operations at hand.
Replace your big table with a much smaller new table. You may have to lock the big table to avoid concurrent writes. Concurrent reads are not a problem.
CREATE TABLE big_tbl_new AS
SELECT b.gid -- or the suggested smaller, faster numeric replacement
, w.word_id, b.stat
FROM big_tbl b
JOIN word w USING (word)
ORDER BY word; -- sorting by word helps query at hand
ORDER BY clusters the data (once) making the query at hand faster, because far fewer blocks have to be read (unless your data is clustered mostly already). The sort carries a cost, weigh cost and benefit once more.
DROP big_tbl; -- make sure your new table has all data!
ALTER big_tbl_new RENAME TO big_tbl;
Recreate indexes:
ALTER TABLE big_tbl ADD CONSTRAINT big_tbl_gid_pkey PRIMARY KEY (gid); -- expendable?
CREATE INDEX big_tbl_word_id_idx ON big_tbl (word_id); -- essential
Your query looks like this now and should be faster:
SELECT b.*
FROM word w
JOIN big_tbl b USING (word_id)
WHERE w.word = 'something';
Reorganization is meant to be a one-time operation to re-organize your data. Keep the new form and also consider keeping indexes permanently.
All of this together (including new indexes) should occupy about half of what you had before on disk, also cutting the time for creation in half (at least). Index creation should be considerably faster, the query as well. If RAM is a limiting factor, these modification pay double.
If you have to write to the table as well, it becomes more expensive (but you did not mention anything about that). You'd need to adjust your logic for DELETE / UPDATE / INSERT:
Example for INSERT: Fetch word_id for existing words or insert a new row in word returning the new word_id. Details for this:
How do I insert a row which contains a foreign key?
Best Answer
No, sorry.
PostgreSQL generally indexes NULL fields, so would it would have to add an entry for each row. You can make an index not index NULLs (using a partial index, or using a non-default type of index which doesn't index NULLs), but it is not implemented for such an index to observe an all-NULL constraint on the column and so take a shortcut of not scanning the table. It would uselessly scan the table looking for not-NULLs anyway. Interesting idea though.