Message-ID: <52B332B3.9050402@optionshouse.com>
Date: Thu, 19 Dec 2013 11:53:55 -0600
From: Shaun Thomas <sthomas@optionshouse.com>
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To: Kai Sellgren <kaisellgren@gmail.com>, <pgsql-performance@postgresql.org>
Subject: Re: Optimizing a query
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On 12/17/2013 08:48 PM, Kai Sellgren wrote:

This is your select:

> SELECT *
> FROM "Log"
> LEFT JOIN "NewsArticle" ON "NewsArticle".id = "Log"."targetId" AND
> "Log"."targetType" = 'NewsArticle'
> ORDER BY "Log"."createdAt" DESC
> LIMIT 10

This is your index:

> CREATE INDEX "Log_targetId_targetType_idx"
>    ON "Log"
>    USING btree
>    ("targetId", "targetType" COLLATE pg_catalog."default");

Unfortunately, this won't help you. You are not matching on any IDs you 
indexed, aside from joining against the article table. You have no WHERE 
clause to restrict the data set, so it absolutely must read the entire 
table to find the most recent records. Without an index on "createdAt", 
how is it supposed to know what the ten most recent records are?

Add an index to the createdAt column:

CREATE INDEX idx_log_createdat ON "Log" (createdAt DESC);

Using that, it should get the ten most recent Log records almost 
immediately, including associated article content.

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-676-8870
sthomas@optionshouse.com

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