Re: Behaviour adding a column with and without a default (prior to PG11)

Hi Tom,

Just thinking about this further, there are other areas where Postgres (correctly, IMO) deviates from the SQL spec and clarifies that in the docs.

For example, https://www.postgresql.org/docs/10/datatype-numeric.html#DATATYPE-NUMERIC-DECIMAL states that for a NUMERIC with no precision specified, "The SQL standard requires a default scale of 0, i.e., coercion to integer precision. We find this a bit useless.", so instead Postgres treats that as arbitrary precision (up to the implementation limit).

Here, the difference in behaviour (ie of adding a nullable column with no default and then changing the default vs adding a nullable with a default) is due to adherence to the spec, but there is no clarifying comment in the docs saying so. Would you consider a documentation patch to clarify this point?

Thanks again,

Joe.

PS. Apologies for the annoying disclaimer on my initial email, I tried to stop it!

On 30/09/2019, 22:19, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

Joe Horsnell <Joe(at)bambooloans(dot)com> writes:
> The documentation (https://www.postgresql.org/docs/10/ddl-alter.html) clearly states that adding a column with a default requires updating all the rows in the table, to store the new column value (this is prior to PG11, obviously).

> Obviously the desired effect of adding the column default without rewriting all the rows in the table can be achieved by adding the nullable column first with no default, then changing the default, but out of curiosity; was there a specific technical reason for the behaviour described above, or was it a conscious design choice?

We read the SQL spec as requiring this behavior.

regards, tom lane

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