Re: Adding an "and is not null" on an indexed field slows the query down immensely.

Try :
order by index_delta+1 desc

On 1/15/11, Tim Uckun <timuckun(at)gmail(dot)com> wrote:
> I have this query it runs reasonably quickly.
>
>
> SELECT "consolidated_urls".* FROM "consolidated_urls" INNER JOIN
> "topical_urls"
> ON "consolidated_urls".id = "topical_urls".consolidated_url_id
> WHERE (("topical_urls".domain_id = 157) AND ((topical_urls.hidden = 'f')))
> ORDER BY index_delta DESC LIMIT 10
>
> The explain for this goes like this.
>
> "Limit (cost=29899.43..29899.46 rows=10 width=1880)"
> " -> Sort (cost=29899.43..29906.04 rows=2642 width=1880)"
> " Sort Key: consolidated_urls.index_delta"
> " -> Nested Loop (cost=101.29..29842.34 rows=2642 width=1880)"
> " -> Bitmap Heap Scan on topical_urls
> (cost=101.29..7490.32 rows=2642 width=4)"
> " Recheck Cond: (domain_id = 157)"
> " Filter: (NOT hidden)"
> " -> Bitmap Index Scan on
> index_topical_urls_on_domain_id_and_consolidated_url_id
> (cost=0.00..100.63 rows=2643 width=0)"
> " Index Cond: (domain_id = 157)"
> " -> Index Scan using consolidated_urls_pkey on
> consolidated_urls (cost=0.00..8.45 rows=1 width=1880)"
> " Index Cond: (consolidated_urls.id =
> topical_urls.consolidated_url_id)"
>
>
> I add one more clause on to it to filter out index_deltas that are not
> null and the query becomes unusably slow.
>
> SELECT "consolidated_urls".* FROM "consolidated_urls" INNER JOIN
> "topical_urls"
> ON "consolidated_urls".id = "topical_urls".consolidated_url_id
> WHERE (("topical_urls".domain_id = 157) AND ((topical_urls.hidden = 'f')))
> AND (index_delta IS NOT NULL) ORDER BY index_delta DESC LIMIT 10
>
> The explain for this is goes like this
>
> "Limit (cost=0.00..20555.33 rows=10 width=1880)"
> " -> Nested Loop (cost=0.00..5430717.58 rows=2642 width=1880)"
> " -> Index Scan Backward using
> index_consolidateds_url_on_index_delta on consolidated_urls
> (cost=0.00..5316175.98 rows=15242 width=1880)"
> " Filter: (index_delta IS NOT NULL)"
> " -> Index Scan using
> index_topical_urls_on_domain_id_and_consolidated_url_id on
> topical_urls (cost=0.00..7.50 rows=1 width=4)"
> " Index Cond: ((topical_urls.domain_id = 157) AND
> (topical_urls.consolidated_url_id = consolidated_urls.id))"
> " Filter: (NOT topical_urls.hidden)"
>
>
>
> The index_delta field is double precision and is indexed.
>
> Any suggestions as to how to make this query run faster?
>
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pasman