Re: [PATCH] Add pg_get_database_ddl() function to reconstruct CREATE DATABASE statement - Mailing list pgsql-hackers

On Wed, 04 Mar 2026 at 16:39, Japin Li <japinli@hotmail.com> wrote:
> On Fri, 27 Feb 2026 at 17:52, Akshay Joshi <akshay.joshi@enterprisedb.com> wrote:
>> Following suggestions from Alvaro and Mark, I have re-implemented this feature using variadic
>> function argument pairs.
>> This patch incorporates Mark Wong’s documentation updates, Amul’s one review comment, and the
>> majority of Euler’s comments.
>>
>> New changes:
>> - Supports flexible DDL options as key-value pairs:
>>     - pretty - Format output for readability
>>     - owner - Include OWNER clause
>>     - tablespace - Include TABLESPACE clause
>>     - defaults - Include parameters even if it is set to default value.
>> - Option values accept: 'yes'/'on'/true/'1' (or their negatives)
>>
>> Usage Examples:
>>   -- Basic usage with options
>>   SELECT pg_get_database_ddl('mydb', 'owner', 'yes', 'defaults', 'yes');
>>   -- Pretty-printed output without owner and tablespace
>>   SELECT pg_get_database_ddl('mydb', 'owner', 'no', 'tablespace', 'no', 'pretty', 'on');
>>   -- Using boolean values
>>   SELECT pg_get_database_ddl('mydb', 'owner', false, 'defaults', true);
>>   -- Using OID
>>   SELECT pg_get_database_ddl(16384, 'pretty', 'yes');
>>
>> Note: To keep things clean, I’ve moved the logic into two generic functions (get_formatted_string
>> and parse_ddl_options) and a common DDLOptionDef struct. This should simplify the work for the
>> rest of the pg_get_<object>_ddl patches.
>>
>> Attached is the v9 patch which is ready for review.
>>
>> On Thu, Feb 26, 2026 at 2:49 AM Euler Taveira <euler@eulerto.com> wrote:
>>
>>  On Wed, Feb 25, 2026, at 8:53 AM, Álvaro Herrera wrote:
>>  >
>>  > I'm surprised to not have seen an update on this topic following the
>>  > discovery by Mark Wong that commit d32d1463995c (in branch 18) already
>>  > established a convention for passing arguments to functions: use argument
>>  > pairs to variadic functions, the way pg_restore_relation_stats() and
>>  > pg_restore_attribute_stats() work.  While I like my previous suggestion
>>  > of using DefElems better, I think it's more sensible to follow this
>>  > established precedent and not innovate on this.
>>  >
>>
>>  This convention is much older than the referred commit. It predates from the
>>  logical decoding (commit b89e151054a0). See pg_logical_slot_get_changes_guts()
>>  that is an internal function for pg_logical_slot_FOO_changes().  It seems a
>>  good idea to have a central function to validate the variadic parameter for all
>>  of these functions.
>>
>
> Thanks for updating the patch, here are some comments on v9.
>
> 1.
> +    uint64        flag;            /* Flag to set */
>
> Do we actually need 64 bits for this flag field?
>
> 2.
> +        /* Indent with spaces */
> +        for (int i = 0; i < nSpaces; i++)
> +        {
> +            appendStringInfoChar(buf, ' ');
> +        }
>
> How about using appendStringInfoSpaces(buf, nSpaces) instead?
>
> 3.
> +    /* If no options provided (VARIADIC NULL), return the empty bitmask */
> +    if (nargs < 0)
> +        return flags;
> +
> +    ...
> +
> +    /* No arguments provided */
> +    if (nargs == 0)
> +        return flags;
>
> These two conditions are identical — how about just `if (nargs <= 0)`?
>
> 4.
> +    /* Arguments must come in name/value pairs */
> +    if (nargs % 2 != 0)
> +        ereport(ERROR,
> +                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
> +                 errmsg("argument list must have even number of elements"),
> +                 errhint("The arguments of %s must consist of alternating keys and values.",
> +                         "pg_get_database_ddl()")));
>
> Should we align this with stats_fill_fcinfo_from_arg_pairs()?
>
> Suggested wording:
>     errmsg("variadic arguments must be name/value pairs")
>
> 5.
> +        /* Key must not be null */
> +        if (nulls[i])
> +            ereport(ERROR,
> +                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
> +                     errmsg("argument %d: key must not be null", i + 1)));
> +
>
> Suggested wording:
>
>     errmsg("name at variadic position %d is null")
>
> 6.
> +        /* Key must be text type */
> +        if (types[i] != TEXTOID)
> +            ereport(ERROR,
> +                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
> +                     errmsg("argument %d: key must be text type", i + 1)));
>
> Suggested wording:
>
>     errmsg("name at variadic position %d has type %s, expected type %s")
>
> 7.
> +            ereport(ERROR,
> +                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
> +                     errmsg("argument %d: value for key \"%s\" must be boolean or text type",
> +                            i + 2, name)));
>
> Suggested wording:
>
>     errmsg("argument \"%s\" has type %s, execpted type boolean or text")
>
> See stats_check_arg_type().
>
> 8.
> +    for (i = 0; i < nargs; i += 2)
> +    {
>
> We can narrow the scope of `i` by declaring it in the for initializer.
>
> 9.
> +        {
> +            bool        found = false;
> +            int         j;
> +
> +            for (j = 0; j < lengthof(ddl_option_defs); j++)
> +            {
>
> Minor style improvements:
>
> - We can (and should) declare `j` inside its for-loop initializer, just like `i`.
> - Move the declaration of `found` up to the top of the outer for-loop scope.
>   This allows us to remove the unnecessary braces around the loop body.
>

After playing with this patch, I’m seeing the following output:

# select pg_get_database_ddl('postgres'::regdatabase, 'defaults', true, 'pretty', true, 'pretty', false);
        pg_get_database_ddl
------------------------------------
 CREATE DATABASE postgres          +
     WITH                          +
         OWNER = japin             +
         ENCODING = 'UTF8'         +
         LC_COLLATE = 'en_US.UTF-8'+
         LC_CTYPE = 'en_US.UTF-8'  +
         COLLATION_VERSION = '2.39'+
         LOCALE_PROVIDER = libc    +
         TABLESPACE = pg_default   +
         ALLOW_CONNECTIONS = true  +
         CONNECTION LIMIT = -1;
(1 row)

Is this the expected behavior?

--
Regards,
Japin Li
ChengDu WenWu Information Technology Co., Ltd.