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" <t...@sss.pgh.pa.us> wrote: Joe Horsnell <j...@bambooloans.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 [Bamboo Limited | 1st Floor | Grenville House | Nelson Gate | Southampton | SO15 1GX ::: www.bambooloans.com] This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. Although, all emails are scanned for viruses, no representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. Help protect our environment by only printing this email if absolutely necessary.
