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help / color / mirror / Atom feedFrom: Tim Dudgeon <tdudgeon.ml@gmail.com>
To: pgsql-sql@postgresql.org
Subject: Re: Querying with arrays
Date: Thu, 04 Dec 2014 10:42:06 -0300
Message-ID: <548064AE.4020407@gmail.com> (raw)
In-Reply-To: <3676.1417100070@sss.pgh.pa.us>
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Looking into this further I don't seem able to get the index used.
I created this simple example:
create table lists (
id SERIAL PRIMARY KEY,
name VARCHAR(32) NOT NULL,
hits INTEGER[] NOT NULL
);
CREATE INDEX idx_lists_hits ON lists USING gin (hits);
INSERT INTO lists (name, hits) VALUES ('list1-10',
ARRAY[1,2,3,4,5,6,7,8,9,10]);
explain analyze SELECT id, name FROM lists
WHERE hits @> array[7];
The plan for the query is this:
"Seq Scan on lists (cost=0.00..16.88 rows=3 width=86) (actual
time=0.006..0.008 rows=1 loops=1)"
" Filter: (hits @> '{7}'::integer[])"
"Planning time: 0.058 ms"
"Execution time: 0.025 ms"
What am I doing wrong?
Tim
On 27/11/2014 11:54, Tom Lane wrote:
> Tim Dudgeon <tdudgeon.ml@gmail.com> writes:
>> I'm considering using arrays to handle managing "lists" of rows (I know
>> this may not be the best approach, but bear with me).
>> I create a table for my lists like this:**
>> create table lists (
>> id SERIAL PRIMARY KEY,
>> hits INTEGER[] NOT NULL
>> );
>> Then I can insert the results of a query into that table as a new list
>> of hits
>> INSERT INTO lists (hits)
>> SELECT array_agg(id)
>> FROM some_table
>> WHERE ...;
>> Now the problem part. How to best use that array of primary key values
>> to restore the data at a later stage. Conceptually I'm wanting this:
>> SELECT * from some_table
>> WHERE id <is in the list of ids in the array in the lists table>;
>> These both work by are really slow:
>> SELECT t1.*
>> FROM some_table t1
>> WHERE t1.id IN (SELECT unnest(hits) from lists WHERE id = 2);
>> SELECT t1.*
>> FROM some_table t1
>> JOIN lists l ON t1.id = any(l.hits)
>> WHERE l.id = 2;
>> Is there an efficient way to do this, or is this a dead end?
> You could create a GIN index on lists.hits and then do
>
> SELECT t1.*
> FROM some_table t1
> JOIN lists l ON array[t1.id] <@ l.hits
> WHERE l.id = 2;
>
> How efficient that will be remains to be determined though;
> if the l.id condition will eliminate a lot of matches it
> could still be kind of slow.
>
> (ISTR some talk of teaching the planner to convert =ANY(array)
> conditions to this form automatically when there's a suitable
> index, but for now you'd have to write it out like this.)
>
> regards, tom lane
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