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0028_how_to_work_with_arrays_part_1.md

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Originally from: tweet, LinkedIn post.

How to work with arrays, part 1

I post a new PostgreSQL "howto" article every day. Join me in this journey – subscribe, provide feedback, share!

Let's talk about arrays today: Docs.

Arrays in Postgres are powerful and existed long before JSON was added. It is worth knowing how to use them to achieve a new level of expressive power (and not just manipulations – you can store them, tables can have columns of array type, even multi-dimensional ones).

If you need to choose just one fact to remember about Postgres arrays, here it is: Postgres is not like others, so array indexing starts with 1, not 0. And don't expect it to complain about an attempt to use 0:

nik=# \pset null '(null)'
Null display is "(null)".

nik=# select (array['one', 'two'])[0];
 array
--------
 (null)
(1 row)

nik=# select (array['one', 'two'])[1];
 array
-------
 one
(1 row)

Creating arrays

Method 1: define array value as a string and converting it to array type

nik=# select '{"one", "two"}'::text[];
   text
-----------
 {one,two}
(1 row)

Method 2: using array[...]

nik=# select array['one', 'two'];
   array
-----------
 {one,two}
(1 row)

Method 3: using select array( <subquery> );

nik=# select array( values('one'), ('two') );
   array
-----------
 {one,two}
(1 row)

This method helps pack many 1-column rows into an array value:

nik=# select array(select pid from pg_stat_activity);
                    array
---------------------------------------------
 {54757,54759,26135,54751,54758,54750,54756}
(1 row)

Interestingly, you can "pack" the whole table into a single value - more precisely, a single-column, single-row table:

nik=# select n from pg_namespace as n \gx
-[ RECORD 1 ]-------------------------------------------------------------------------
n | (99,pg_toast,10,)
-[ RECORD 2 ]-------------------------------------------------------------------------
n | (11,pg_catalog,10,"{nik=UC/nik,=U/nik}")
-[ RECORD 3 ]-------------------------------------------------------------------------
n | (2200,public,6171,"{pg_database_owner=UC/pg_database_owner,=U/pg_database_owner}")
-[ RECORD 4 ]-------------------------------------------------------------------------
n | (13471,information_schema,10,"{nik=UC/nik,=U/nik}")
-[ RECORD 5 ]-------------------------------------------------------------------------
n | (41022,pgss_pgsa,10,)

nik=# select array(select n from pg_namespace as n) \gx
-[ RECORD 1 ]------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
array | {"(99,pg_toast,10,)","(11,pg_catalog,10,\"{nik=UC/nik,=U/nik}\")","(2200,public,6171,\"{pg_database_owner=UC/pg_database_owner,=U/pg_database_owner}\")","(13471,information_schema,10,\"{nik=UC/nik,=U/nik}\")","(41022,pgss_pgsa,10,)"}

This value retains column names, and you can unpack it back, using unnest(..) and (t).*:

nik=# with packed(val) as (
  select array(select n from pg_namespace as n)
)
select unnest(val)
from packed;
                                       unnest
------------------------------------------------------------------------------------
 (99,pg_toast,10,)
 (11,pg_catalog,10,"{nik=UC/nik,=U/nik}")
 (2200,public,6171,"{pg_database_owner=UC/pg_database_owner,=U/pg_database_owner}")
 (13471,information_schema,10,"{nik=UC/nik,=U/nik}")
 (41022,pgss_pgsa,10,)
(5 rows)

nik=# with packed(val) as (
  select array(select n from pg_namespace as n)
)
select (unnest(val)).*
from packed;
  oid  |      nspname       | nspowner |                            nspacl
-------+--------------------+----------+---------------------------------------------------------------
    99 | pg_toast           |       10 | (null)
    11 | pg_catalog         |       10 | {nik=UC/nik,=U/nik}
  2200 | public             |     6171 | {pg_database_owner=UC/pg_database_owner,=U/pg_database_owner}
 13471 | information_schema |       10 | {nik=UC/nik,=U/nik}
 41022 | pgss_pgsa          |       10 | (null)
(5 rows)

Magic! This can help you write very complex CTEs. (It is recommended to always comment each step though – Postgres supports SQL-standard -- commenting, as well as /* ... */.)

Method 4: aggregation functions

nik=# select array_agg(i) from generate_series(1, 3) as i;
 array_agg
-----------
 {1,2,3}
(1 row)

This method can also be used to pack all single-column rows into one array value:

nik=# select array_agg(pid) from pg_stat_activity;
                  array_agg
---------------------------------------------
 {54757,54759,26135,54751,54758,54750,54756}
(1 row)

How to access array elements

Accessing array members is pretty straightforward:

nik=# select (array_agg(pid))[1] from pg_stat_activity;
 array_agg
-----------
     54757
(1 row)

Again, don't forget that indexes start with 1, and accessing a non-existent member won't produce an error, it will return NULL:

nik=# select (array_agg(pid))[100000] from pg_stat_activity;
 array_agg
-----------
    (null)
(1 row)

You can use arr[N:M] to get only a "slice" of array. And you can omit one of the numbers – for example, to keep only the first 3 members, use arr[:3]:

nik=# select (array_agg(pid))[:3] from pg_stat_activity;
 array_agg
---------------------
 {54757,54759,26135}
(1 row)

// To be continued