Horton

Re: remaining sql/json patches

amit <amitlangote09@gmail.com>

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From: Amit Langote <amitlangote09@gmail.com>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Erik Rijkers <er@xs4all.nl>, jian he <jian.universality@gmail.com>, Andrew Dunstan <andrew@dunslane.net>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2023-09-14T08:14:51Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. SQL/JSON: Various improvements to SQL/JSON query function docs

  2. SQL/JSON: Fix some obsolete comments.

  3. SQL/JSON: Fix issues with DEFAULT .. ON ERROR / EMPTY

  4. JSON_TABLE: Add support for NESTED paths and columns

  5. Fix JsonExpr deparsing to emit QUOTES and WRAPPER correctly

  6. Fix typo introduced in 6185c9737

  7. Add basic JSON_TABLE() functionality

  8. Avoid splitting errmsg string to span multiple lines

  9. Add SQL/JSON query functions

  10. Implement various jsonpath methods

  11. Add soft error handling to some expression nodes

  12. Adjust populate_record_field() to handle errors softly

  13. Refactor code used by jsonpath executor to fetch variables

  14. Test EXPLAIN (FORMAT JSON) ... XMLTABLE

  15. Simplify productions for FORMAT JSON [ ENCODING name ]

  16. Add trailing commas to enum definitions

  17. doc: add missing <returnvalue> and whitespace

  18. Add more SQL/JSON constructor functions

  19. Rename a nonterminal used in SQL/JSON grammar

  20. Some refactoring to export json(b) conversion functions

  21. Don't include CaseTestExpr in JsonValueExpr.formatted_expr

  22. Code review for commit b6e1157e7d

  23. Pass constructName to transformJsonValueExpr()

  24. Unify JSON categorize type API and export for external use

  25. Make some indentation in gram.y consistent

  26. Allow most keywords to be used as column labels without requiring AS.

  27. Reduce size of backend scanner's tables.

  28. Use perfect hashing, instead of binary search, for keyword lookup.

Attachments

Thanks for the review.

On Thu, Sep 7, 2023 at 12:01 AM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> 0001 is quite mysterious to me.  I've been reading it but I'm not sure I
> grok it, so I don't have anything too intelligent to say about it at
> this point.  But here are my thoughts anyway.
>
> Assert()ing that a pointer is not null, and in the next line
> dereferencing that pointer, is useless: the process would crash anyway
> at the time of dereference, so the Assert() adds no value.  Better to
> leave the assert out.  (This appears both in ExecExprEnableErrorSafe and
> ExecExprDisableErrorSafe).
>
> Is it not a problem to set just the node type, and not reset the
> contents of the node to zeroes, in ExecExprEnableErrorSafe?  I'm not
> sure if it's possible to enable error-safe on a node two times with an
> error reported in between; would that result in the escontext filled
> with junk the second time around?  That might be dangerous.  Maybe a
> simple cross-check is to verify (assert) in ExecExprEnableErrorSafe()
> that the struct is already all-zeroes, so that if this happens, we'll
> get reports about it.  (After all, there are very few nodes that handle
> the SOFT_ERROR_OCCURRED case).
>
> Do we need to have the ->details_wanted flag turned on?  Maybe if we're
> having ExecExprEnableErrorSafe() as a generic tool, it should receive
> the boolean to use as an argument.
>
> Why palloc the escontext always, and not just when
> ExecExprEnableErrorSafe is called?  (At Disable time, just memset it to
> zero, and next time it is enabled for that node, we don't need to
> allocate it again, just set the nodetype.)
>
> ExecExprEnableErrorSafe() is a strange name for this operation.  Maybe
> you mean ExecExprEnableSoftErrors()?  Maybe it'd be better to leave it
> as NULL initially, so that for the majority of cases we don't even
> allocate it.

I should have clarified earlier why the ErrorSaveContext must be
allocated statically during the expression compilation phase. This is
necessary because llvm_compile_expr() requires a valid pointer to the
ErrorSaveContext to integrate into the compiled version. Thus, runtime
allocation isn't feasible.

After some consideration, I believe we shouldn't introduce the generic
ExecExprEnable/Disable* interface. Instead, we should let individual
expressions manage the ErrorSaveContext that they want to use
directly, using ExprState.escontext just as a temporary global
variable, much like ExprState.innermost_caseval is used.

The revised 0001 now only contains the changes necessary to make
CoerceViaIO evaluation code support soft error handling.

> In 0002 you're adding soft-error support for a bunch of existing
> operations, in addition to introducing SQL/JSON query functions.  Maybe
> the soft-error stuff should be done separately in a preparatory patch.

Hmm, there'd be only 1 ExecExprEnableErrorSafe() in 0002 -- that in
ExecEvalJsonExprCoercion().  I'm not sure which others you're
referring to.

Given what I said above, the code to reset the ErrorSaveContext
present in 0002 now looks different.  It now resets the error_occurred
flag directly instead of using memset-0-ing the whole struct.
details_wanted and error_data are both supposed to be NULL in this
case anyway and remain set to NULL throughout the lifetime of the
ExprState.

> I think functions such as populate_array_element() that can now save
> soft errors and which currently do not have a return value, should
> acquire a convention to let caller know that things failed: maybe return
> false if SOFT_ERROR_OCCURRED().  Otherwise it appears that, for instance
> populate_array_dim_jsonb() can return happily if an error occurs when
> parsing the last element in the array.  Splitting 0002 to have a
> preparatory patch where all such soft-error-saving changes are
> introduced separately would help review that this is indeed being
> handled by all their callers.

I've separated the changes to jsonfuncs.c into an independent patch.
Upon reviewing the code accessible from populate_record_field() --
which serves as the entry point for the executor via
json_populate_type() -- I identified a few more instances where errors
could be thrown even with a non-NULL escontext. I've included tests
for these in patch 0003. While some error reports, like those in
construct_md_array() (invoked by populate_array()), fall outside
jsonfuncs.c, I assume they're deliberately excluded from SQL/JSON's ON
ERROR support. I've opted not to modify any external interfaces.

--
Thanks, Amit Langote
EDB: http://www.enterprisedb.com