Re: Support logical replication of DDLs

On 2022-Oct-31, Peter Smith wrote:

> 6. add_policy_clauses
>
> + else
> + {
> + append_bool_object(policyStmt, "present", false);
> + }
>
> Something seems strange. Probably I'm wrong but just by code
> inspection it looks like there is potential for there to be multiple
> param {present:false} JSON objects:
>
> {"present" :false},
> {"present" :false},
> {"present" :false},
>
> Shouldn't those all be array elements or something? IIUC apart from
> just DDL, the JSON idea was going to (in future) allow potential
> machine manipulation of the values prior to the replication, but
> having all these ambiguous-looking objects does not seem to lend
> itself to that idea readily. How to know what are each of those params
> representing?

Do you mean that a single JSON object has multiple member with
"present":"false"? That sounds like something we should never produce,
and post-processing to remove them does not sound good either. Is that
really what is happening, or do I misunderstand?

Obviously, if you have an object with several sub-objects, each of the
sub-objects can have its own "present:false" label. The idea is that
the clause that each subobject represents may not be in the command as
written by the user; but perhaps a post-processor of the JSON blob wants
to change things so that the clause does appear in the final output.
And this should be doable for each individual optional clause in each
command, which means that, yeah, there should be multiple
"present:false" pairs in a single JSON blob, in different paths.

(For example, if the user writes "CREATE SEQUENCE foobar", we would get
a tree that has {fmt: "CACHE %{value}", present: false, value: 32}, so
if you just convert that to text DDL without further ado you would get
the original command verbatim; but you can poke the "present" to true so
you would get "CREATE SEQUENCE foobar CACHE 32".)

Also, I think I came up with the idea of having "present:boolean" a bit
late in the development of this code, so it's quite possible that there
are commands that are inconsistent in their support of this pattern.
That makes it especially important to review the representation of each
command carefully.

--
Álvaro Herrera 48°01'N 7°57'E — https://www.EnterpriseDB.com/
"Postgres is bloatware by design: it was built to house
PhD theses." (Joey Hellerstein, SIGMOD annual conference 2002)