Re: BUG #18138: Using limit on VALUES causes type conversion to fail.

On Thu, Sep 28, 2023 at 1:48 PM PG Bug reporting form <noreply@postgresql.org> wrote:

The following bug has been logged on the website:

Bug reference:      18138
Logged by:          Akash Kava
Email address:      ackava@gmail.com
PostgreSQL version: 15.4
Operating system:   alpine
Description:       

Doing UPSERT is not easy and I know it is a complicated process. However,
since UPDATE has a WHERE clause, there is no way we can do conditional
INSERT.

But I was able to find a workaround, using LIMIT keyword with VALUES. I am
aware of the ON CONFLICT clause, but the issue with ON CONFLICT is, it
increases identity every time we want to update a row.

 INSERT INTO the_table(column_1, column_2)
      VALUES ($1, $2)
      LIMIT LEAST((SELECT 1 FROM the_table WHERE key_1 = $3),2)-1

Basically this is a part of a larger query I have explained here.
https://stackoverflow.com/a/77190090/85597

This works as expected except for boolean and date fields.

Here is the example,
https://www.db-fiddle.com/f/g4LMVToHjrbYTDXT4MB1K/2

insert into avatar(username,url,is_public)
values ('a','b', 'true');

insert into avatar(username,url)
values ('a1','b')
limit LEAST((SELECT 1 FROM avatar),2)-1;

insert into avatar(username,url,is_public)
values ('a2','b', 'true')
limit LEAST((SELECT 1 FROM avatar),2)-1;

The problem occurs when we are sending data from node-postgres library.

Is there any work around? IF this will work correctly, we will be able to
use UPSERT easily without having gaps in identity sequences

A smaller example would be that this works and inserts the row:

    insert into avatar(username, url, is_public)
    values ('a','b', 'true');

while you get the error with:

    insert into avatar(username, url, is_public)
    values ('a2','b', 'true')
    limit 1 ;

I am not sure if this would be classified as a bug since you are putting quotes around the boolean value. 

Without quotes it would work fine:

    insert into avatar(username, url, is_public)
    values ('a1','b', true)
    limit 1 ;

or if you explicitly converted to the type of the column:

    insert into avatar(username, url, is_public)
    values ('a2','b', 'true'::boolean)
    limit 1 ;

Best regards

Pantelis Theodosiou