The query planner in sqlite3 < 3.8 is not very clever, so when it sees
num mentioned in the query filter, it decides not to use the fast idx_ts
index and goes for the much slower autoindex. CentOS-7 still has
sqlite-3.7, so loading the http landing page of a very large archive
(LKML) was taking over 18 seconds, as oppposed to milliseconds on a
system with sqlite-3.8 and above:
$ time sqlite3 -line over.sqlite3 'SELECT ts,ds,ddd FROM over \
WHERE num > 0 ORDER BY ts DESC LIMIT 1000;' > /dev/null
real 0m19.610s
user 0m17.805s
sys 0m1.805s
$ sqlite3 -line over.sqlite3 'EXPLAIN QUERY PLAN SELECT ts,ds,ddd \
FROM over WHERE num > 0 ORDER BY ts DESC LIMIT 1000;'
selectid = 0
order = 0
from = 0
detail = SEARCH TABLE over USING INDEX sqlite_autoindex_over_1 (num>?) (~250000 rows)
However, if we slightly tweak the query per SQlite recommendations [1]
by adding + to the num filter, we force it to use the correct index
and see much faster performance:
$ time sqlite3 -line over.sqlite3 'SELECT ts,ds,ddd FROM over \
WHERE +num > 0 ORDER BY ts DESC LIMIT 1000;' > /dev/null
real 0m0.007s
user 0m0.005s
sys 0m0.002s
$ sqlite3 -line over.sqlite3 'EXPLAIN QUERY PLAN SELECT ts,ds,ddd \
FROM over WHERE +num > 0 ORDER BY ts DESC LIMIT 1000;'
selectid = 0
order = 0
from = 0
detail = SCAN TABLE over USING INDEX idx_ts (~
1464303 rows)
This appears to be the only place where this is needed in order to avoid
running into this issue.
As far as I can tell, this change has no impact on systems running newer
sqlite3 (>= 3.8).
.. [1] https://sqlite.org/optoverview.html#disqualifying_where_clause_terms_using_unary_
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
my ($s, @v);
if (defined($before)) {
if (defined($after)) {
- $s = 'num > 0 AND ts >= ? AND ts <= ? ORDER BY ts DESC';
+ $s = '+num > 0 AND ts >= ? AND ts <= ? ORDER BY ts DESC';
@v = ($after, $before);
} else {
- $s = 'num > 0 AND ts <= ? ORDER BY ts DESC';
+ $s = '+num > 0 AND ts <= ? ORDER BY ts DESC';
@v = ($before);
}
} else {
if (defined($after)) {
- $s = 'num > 0 AND ts >= ? ORDER BY ts ASC';
+ $s = '+num > 0 AND ts >= ? ORDER BY ts ASC';
@v = ($after);
} else {
- $s = 'num > 0 ORDER BY ts DESC';
+ $s = '+num > 0 ORDER BY ts DESC';
}
}
my $msgs = do_get($self, <<"", $opts, @v);