Re: SQLite, UPDATE, and LIMIT

[Eric Wong <e@80x24.org>]

Jonathan Corbet <corbet@lwn.net> wrote:
> So I'm messing with public-inbox a bit.  I had to make the following
> change before things would work on my Fedora system.  (This is with
> v1.0.0).
> 
> The symptom was errors out of SQLite; it wouldn't pass "make test" much
> less work properly.  Some digging turns up this:
> 
> 	If SQLite is built with the SQLITE_ENABLE_UPDATE_DELETE_LIMIT
> 	compile-time option then the syntax of the UPDATE statement is
> 	extended with optional ORDER BY and LIMIT clauses as follows
> 
> Evidently, Fedora does not enable that option.

Odd; I didn't know about that option and don't know if there's
performance implications for this.

So with your patch, is everything else good on Fedora?

> Now, I assume that LIMIT is there for a reason; I've not even tried to
> look at the schema that public-inbox uses yet.  But I thought I would toss
> this out there to see you thought.  Perhaps what I really need to do is to
> ask Fedora to turn that option on?

Using LIMIT is probably just a habit of mine.

> --- a/lib/PublicInbox/Msgmap.pm
> +++ b/lib/PublicInbox/Msgmap.pm
> @@ -57,7 +57,7 @@ sub meta_accessor {
>  	$prev = $dbh->selectrow_array($sql, undef, $key);
>  
>  	if (defined $prev) {
> -		$sql = 'UPDATE meta SET val = ? WHERE key = ? LIMIT 1';
> +		$sql = 'UPDATE meta SET val = ? WHERE key = ?';

`key' is the primary key anyways, so it should be fine to remove
LIMIT, there.  Will apply.

>  		$dbh->do($sql, undef, $value, $key);
>  	} else {
>  		$sql = 'INSERT INTO meta (key,val) VALUES (?,?)';