I would need double check if overlaps uses an index. 

If it does you can use explain to figure out which table is being fill y 
scanned and adjust the order in the statement accordingly. 

If it doesn't, then you can use something that does (e.g. &&) and ensure that 
the smaller table is the one being scanned.

Jim

On July 15, 2016 9:15:55 AM EDT, Richard McDonnell <richard.mcdonn...@opw.ie> 
wrote:
>Need a little help, this may not be the forum, but I thought I would 
>throw it out there!
>I am looking to minimise the search area in this query
>
>SELECT objectid
>FROM
>level_1_20151206 AS A, water_polygon AS Z
>WHERE
>(SELECT ST_Overlaps (A.the_geom, Z.geom));
>
>What I want to do is use the extent of A's geometry to limit the search
>
>for overlaps with Z if that makes sense!?
>These are both large National coverage datasets.
>Any help suggestions welcomed!!
>
>Regards,
>
>Richard.
>
>
>
>OPW - Ag féachaint don am atá le teacht - Ag caomhnú ón am atá thart
>OPW - Looking to the future - Caring for the past
>
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