Is it better to use tables instead of arrays field type in PostgreSql when arrays do not exceed 50 elements?

I avoid arrays for 2 reasons:

  • by storing more than one attribute value in a cell you violate the first normal form (theoretical);
  • you have to perform some extra, non-SQL related, processing each time you need to work with individual elements of the arrays (practical, but a direct consequence of the theoretical one)

I've considered this problem as well and the conclusion that I came to, is to use arrays when you want to eliminate table joins. The number of elements contained in each array isn't as important as the size of the tables involved. If there are only a few thousand rows in each table, then joining to get the 50 sub rows shouldn't be a big problem. If you get into 10's or 100's of thousands or rows, you're likely to start chewing through a lot of processor time and disk i/o though.


Don't know how long these links stay live so I'll paste the results below: http://sqlfiddle.com/#!17/55761/2

TLDR; searching a table index and then joining is fast, BUT adding a GIN index (using gin__int_ops) to a single table with an array column can be faster. Additionally, the flexibility of being able to match "some" or a small number of your array values might be a better option e.g. a tagging system.

create table data (
    id serial primary key,
    tags int[],
    data jsonb
);

create table tags (
    id serial primary key,
    data_id int references data(id)
);

CREATE INDEX gin_tags ON data USING GIN(tags gin__int_ops); 

SET enable_seqscan to off;

with rand as (SELECT generate_series(1,100000) AS id)
insert into data (tags) select '{5}' from rand;

update data set tags = '{1}' where id = 47300;

with rand as (SELECT generate_series(1,100000) AS id)
INSERT INTO tags(data_id) select id from rand;

Running:

  select data.id, data.data, data.tags
  from data, tags where tags.data_id = data.id and tags.id = 47300;

and

  select data.id, data.data, data.tags
  from data where data.tags && '{1}';

Yields:

Record Count: 1; Execution Time: 3ms
QUERY PLAN
Nested Loop (cost=0.58..16.63 rows=1 width=61)
-> Index Scan using tags_pkey on tags (cost=0.29..8.31 rows=1 width=4)
Index Cond: (id = 47300)
-> Index Scan using data_pkey on data (cost=0.29..8.31 rows=1 width=61)
Index Cond: (id = tags.data_id)

and

Record Count: 1; Execution Time: 1ms
QUERY PLAN
Bitmap Heap Scan on data (cost=15.88..718.31 rows=500 width=61)
Recheck Cond: (tags && '{1}'::integer[])
-> Bitmap Index Scan on gin_tags (cost=0.00..15.75 rows=500 width=0)
Index Cond: (tags && '{1}'::integer[])

Tags:

Arrays

Postgresql

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