Re: Why my cursor construction is so slow?

# kleptog@svana.org / 2006-06-22 09:19:44 +0200:
> On Tue, Jun 20, 2006 at 02:06:19AM -0700, biuro@globeinphotos.com wrote:
> > Such construction is very slow but when I modify SQL to:
> >         OPEN cursor1 FOR SELECT * FROM alias WHERE mask>=alias_out
> > ORDER BY mask LIMIT 100;
> > 
> > it works very fast. It is strange for me becuase I've understood so far
> > that when cursor is open select is executed but Postgres does not
> > select all rows - only cursor is positioned on first row, when you
> > execute fetch next row is read. But this example shows something
> > different.
> 
> PostgreSQL tries to optimise for overall query time. Without the limit
> it tries to find a plan that will return the whole set as quick as
> possible.

    That looks like the wrong approach for a cursor.

> With the LIMIT it might take a different approach, which
> might be worse if you read the whole lot, but better for a limited set.
> A fast-start plan so to speak.

    That looks like a better approach for a cursor.

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