How to make an array of ids in postgres? [duplicate]

In addition to violating First Normal Form because of the repeating group of values stored in a single column, comma-separated lists have a lot of other more practical problems:

• Can’t ensure that each value is the right data type: no way to prevent 1,2,3,banana,5
• Can’t use foreign key constraints to link values to a lookup table; no way to enforce referential integrity.
• Can’t enforce uniqueness: no way to prevent 1,2,3,3,3,5
• Can’t delete a value from the list without fetching the whole list.
• Can't store a list longer than what fits in the string column.
• Hard to search for all entities with a given value in the list; you have to use an inefficient table-scan. May have to resort to regular expressions, for example in MySQL:
  idlist REGEXP '[[:<:]]2[[:>:]]' or in MySQL 8.0: idlist REGEXP '\\b2\\b'
• Hard to count elements in the list, or do other aggregate queries.
• Hard to join the values to the lookup table they reference.
• Hard to fetch the list in sorted order.
• Hard to choose a separator that is guaranteed not to appear in the values

To solve these problems, you have to write tons of application code, reinventing functionality that the RDBMS already provides much more efficiently.

Comma-separated lists are wrong enough that I made this the first chapter in my book: SQL Antipatterns: Avoiding the Pitfalls of Database Programming.

There are times when you need to employ denormalization, but as @OMG Ponies mentions, these are exception cases. Any non-relational “optimization” benefits one type of query at the expense of other uses of the data, so be sure you know which of your queries need to be treated so specially that they deserve denormalization.

"One reason was laziness".

This rings alarm bells. The only reason you should do something like this is that you know how to do it "the right way" but you have come to the conclusion that there is a tangible reason not to do it that way.

Having said this: if the data you are choosing to store this way is data that you will never need to query by, then there may be a case for storing it in the way you have chosen.

(Some users would dispute the statement in my previous paragraph, saying that "you can never know what requirements will be added in the future". These users are either misguided or stating a religious conviction. Sometimes it is advantageous to work to the requirements you have before you.)

There are numerous questions on SO asking:

• how to get a count of specific values from the comma separated list
• how to get records that have only the same 2/3/etc specific value from that comma separated list

Another problem with the comma separated list is ensuring the values are consistent - storing text means the possibility of typos...

These are all symptoms of denormalized data, and highlight why you should always model for normalized data. Denormalization can be a query optimization, to be applied when the need actually presents itself.