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Thread: Optimizing a query

Optimizing a query

From
Kai Sellgren
Date:
Hi,

I'm new to PostgreSQL and trying to run this query:

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

Basically I'm finding the last 10 log entries, which point (targetType) to news articles.

The explain analyze is this:

http://d.pr/i/mZhl (I didn't know how to copy from the pgAdmin, without having a huge mess)

I have this index on Log:

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

I have ran Vacuum and Analyze on both tables.

What am I missing here?


--
Yours sincerely,
Kai Sellgren

Re: Optimizing a query

From
Shaun Thomas
Date:
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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Re: Optimizing a query

From
Mark Kirkwood
Date:
On 20/12/13 06:53, Shaun Thomas wrote:
> 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.
>

Also, might be worth creating an index on NewsArticle(id) so that the
join to this table does not require a full table scan:

CREATE INDEX newsarticle_id_idx ON "NewsArticle" (id);

(probably not a problem when you only have a few articles - but will be
as the volume increases over time).

Regards

Mark