Re: deparsing utility commands

Executive summary:

There is now a CommandDeparse_hook;
deparse_utility_command is provided as an extension, intended for 9.6;
rest of patch would be pushed to 9.5.


Long version:

I've made command deparsing hookable.  Attached there are three patches:
the first patch contains changes to core that just add the "command
list" stuff, so that on ddl_command_end there is access to what has been
executed.  This includes the OID of the object just created, the command
tag, and assorted other details; the deparsed command in JSON format is
not immediately part of the result.

The third patch contains all the deparse code, packaged as a contrib
module and extension named ddl_deparse.  Essentially, it's everything
that was previously in tcop/deparse_utility.c and utils/adt/ddl_json.c:
the stuff that takes the parsenode and OID of a command execution and
turns it into a JSON blob, and also the support function that takes the
JSON blob and converts back into the plain text rendering of the
command.

The second patch contains some changes to core code that support the
ddl_deparse extension; mainly some ruleutils.c changes.

What links patches 0001 and 0003 is a hook, CommandDeparse_hook.  If
unset, the pg_event_trigger_ddl_commands function returns some
boilerplate text like "no deparse function installed"; if the extension
is installed, the JSON rendering is returned instead and can be used
with the ddl_deparse_expand_command() function.

The rationale for doing things this way is that it will be useful to
have 9.5 expose the pg_event_trigger_ddl_commands() function for various
uses, while we refine the JSON bits some more and get it committed for
9.6.  In reviews, it's clear that there's some more bits to fiddle so
that it can be as general as possible.  I think we should label the
whole DDL command reporting as experimental in 9.5 and subject to
change, so that we can just remove the hook in 9.6 when the ddl_deparse
thing becomes part of core.

Thoughts?

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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