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Re: Using Sqlite in libsvn_wc towards Subversion 1.9++

From: Branko Čibej <brane_at_wandisco.com>
Date: Sun, 17 Nov 2013 14:08:27 +0100

On 17.11.2013 13:34, Bert Huijben wrote:
> Hi,
>
> For Subversion 1.8 I spend a lot of time optimizing our use of Sqlite in the
> working copy to work as optimal as possible with the then Supported range of
> Sqlite versions (3.7.12-3.7.19). Then a few months later in Sqlite 3.8.0 a
> new query planner was introduced that broke quite a few of the optimizations
> I/we assumed were ok, because we did things different than the Sqlite
> developers intended.
>
> For Subversion 1.8 we did:
> * Try to support as many sqlite versions as possible (3.7.12-infinity)
> * Try to optimize queries without describing our layout
> * Add many columns to indexes to make them unique and avoid going back to
> the original table when possible.
>
> This goes against +- all the recommendations of the Sqlite designers as they
> recommend
> * Provide your users exactly the sqlite version you tested against -> link
> statically as part of your source
> * Provide statistics about your table.
> * Use 'good indexes' (either with stats; or with a good first key)
>
> As users that have upgraded to Sqlite 3.8.0 or later have found out our
> design choices really affect performance on huge working copies: several
> performance critical queries during operations such as commit, delete and
> revert are now handled as a full table scan.
>
>
> To optimize our queries for Subversion 1.7 and 1.8 we should probably write
> a bit of code to fill the sqlite statistics table with the information
> Sqlite needs to make the right decisions,

I've looked at pre-filling the sqlite_stats1 table. I think it's not
going to work because apparently some of the stats columns contain index
row counts, which of course vary not only from one working copy to the
next, but also during the lifetime of a working copy. Maybe there's a
way to fake "infinite" size indexes; I haven't found documentation on
how to do that. Most of the analyze results seem to be concerned with
the wc_id column.

As a data point: running ANALYZE in a wc.db for a checkout of
^/subversion takes about half a minute on my mac (with SSD, fwiw), so I
don't think re-running it incrementally is an option.

> but for Subversion 1.9 and later
> we should decide what we want to do to make sure our users will never see
> such a regression any more.
>
> I think we should at least:
> * Update our indexes to be closer to what the Sqlite developers call 'good'
> indexes': remove unnecessary columns.
> (In the sqlite implementation they are already unique via the hidden rowid)
>
> *****
> But I would also like to recommend/ask that we start bundling Sqlite with
> Subversion, to allow optimizing for the specific version we use for a
> release without risking future breakage.
> *****

You realize that if we do that, we also have to provide a sqlite
command-line tool based on that same version.

There's another option: that we use different indexes based on the
version of SQLite that the wc.db was created with. We can always detect
when the sqlite binaries are downgraded (or upgraded), and could emit an
error or warning; and we could provide 'svn cleanup --reindex', or some
equivalent, that would rebuild indexes according to the current SQLite
version, without having to throw away the working copy and start over.

> I don't expect that our *nix packagers that like to package every component
> separately are going to like this, but I think we should do this to
> * guarantee our users a reasonably performance.
> * to allow us to diagnose user performance problems
> Perhaps a few will even patch their own sqlite version in, but then we can
> safely say that we didn't cause the performance problems users have in these
> installations.

Whatever we do, we should always allow linking to external SQLite. The
most we can do, IMO, is to not search for the headers and libs unless
--with-sqlite was given explicitly during configure time.

> If we do that and/or bump our requirement to the soon to be released Sqlite
> 3.8.2 (or later) we can use quite a few new indexing tricks that should
> improve update performance on the NODES table and the always attached
> triggers more than a bit.*)
>
>
>
> If we can get consensus around this I would like to add the Sqlite 3.8.2
> amalgamation directly to our source tree about a week after its release
> (Probably December 2013). We can than start optimizing our Sqlite schema and
> query usage towards this version.
>
>
> Bert
>
> *)
> * The PRISTINE table is the perfect candidate for a 'WITHOUT rowid' table,
> which should give a 30% performance boost on update/select. (Requires Sqlite
> 3.8.2)
> * The moved_to index on nodes is a typical example of where a partial index
> would help reducing unnecessary index updates (Requires 3.8.0).

As a matter of fact, all indexes that begin with wc_id are candidates
for partial indexing (with WHERE wc_id=1).

-- Brane

-- 
Branko Čibej | Director of Subversion
WANdisco // Non-Stop Data
e. brane_at_wandisco.com
Received on 2013-11-17 14:21:46 CET

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