Hello,
[sqlite 3.5.8, Mac OS X 10.5.2]
Given the following schema [1]:
document <->> document_token <<-> token
I'm trying to retrieve some documents with a set of token(s) [2].
Things works rather fine when looking for multiple tokens:
select sum( document_token.weight ) as score,
document.name as name
from (
select id
from token
where name >= 'america' and name < 'americb'
union
select id
from token
where name >= 'america' and name < 'americb'
) as token
join document_token on document_token.token_id = token.id
join document on document.id = document_token.document_id
group by document.name
having count( * ) >= 2
order by 1 desc,
2
limit 5;
0|0|TABLE token WITH INDEX token_name
0|0|TABLE token WITH INDEX token_name
0|0|TABLE AS token
1|1|TABLE document_token WITH INDEX document_token_token_id
2|2|TABLE document USING PRIMARY KEY
CPU Time: user 0.130602 sys 0.073889
Unfortunately, with only one token value, the execution time goes
through the roof:
select sum( document_token.weight ) as score,
document.name as name
from (
select id
from token
where name >= 'america' and name < 'americb'
) as token
join document_token on document_token.token_id = token.id
join document on document.id = document_token.document_id
group by document.name
having count( * ) >= 1
order by 1 desc,
2
limit 5;
0|2|TABLE document WITH INDEX document_name ORDER BY
1|1|TABLE document_token WITH INDEX document_token_document_id
2|0|TABLE token USING PRIMARY KEY
CPU Time: user 14.673524 sys 1.378078
This seems to be due to the fact that in the second case SQLite will
favor the document table to lead the query, resulting in pretty much a
full scan of the entire database as the restricting qualifier is on
the token table, not the document.
What would be a reasonable way to make those two queries use a similar
execution plan (e.g. token -> document_token -> document instead of
document -> document_token -> token)?
Thanks in advance.
Kind regards,
--
PA.
http://alt.textdrive.com/nanoki/
[1] http://dev.alt.textdrive.com/browser/HTTP/Finder.ddl
[2] http://dev.alt.textdrive.com/browser/HTTP/Finder.dml#L70
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