2011/5/16 Branko Čibej <br...@e-reka.si>: > On 16.05.2011 03:13, Hyrum K Wright wrote: >> Several places in wc_db we use the following pattern to select all >> nodes with a common tree ancestor: >> WHERE wc_id = ?1 AND (local_relpath = ?2 OR local_relpath LIKE ?3 ESCAPE >> '#') >> >> While this works, there was some concern about whether or not SQLite >> was using the proper indicies when executing this query. By examining >> the output for 'EXPLAIN QUERY PLAN' on some of the relevant SELECT >> statements, I believe it does use the indicies as intended. >> >> However, I stumbled across an alternate implementation which I believe >> has some merit. Instead of the above clause, we could use: >> WHERE wc_id = ?1 AND substr(local_relpath, 1, length(?2)) = ?2 >> >> This also avoids a table scan by making use of the indicies, but has >> the advantage of not having to compute a separate parameter for the >> LIKE clause in C. It returns the same results, and has the benefit of >> being a bit more clear to SQLite what we're trying to accomplish. I'm >> tempted to switch our code to using this new format, but wanted some >> comments first. I have not yet run extensive timing or other analysis >> on the performance. >> >> Thoughts? >> >> -Hyrum > > Can't be right. I'm assuming the first query works correctly iff: > ?2 = foo > ?3 = foo/% > > and returns 'foo' and all its subtree. > > The second query can't return the same results; if ?2=foo, it'll match > foobar, which is not foo's child; if ?2=foo/, it won't return foo.
That's what I get for writing mail at 3am. I believe the following would fix this: WHERE wc_id = ?1 AND (local_relpath = ?2 OR substr(local_relpath, 1, length(?2 + 1)) = ?2 || '/') -Hyrum
