Horton

Re: Optimizing nbtree ScalarArrayOp execution, allowing multi-column ordered scans, skip scan

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Tomas Vondra <tomas.vondra@enterprisedb.com>, Jeff Davis <pgsql@j-davis.com>, benoit <benoit@hopsandfork.com>, Alexander Korotkov <aekorotkov@gmail.com>
Date: 2024-04-08T01:11:48Z
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. Move nbtree preprocessing into new .c file.

  2. Fix nbtree lookahead overflow bug.

  3. Remove unneeded nbtree array preprocessing assert.

  4. Don't try to fix eliminated nbtree array scan keys.

  5. Remove redundant nbtree preprocessing assertions.

  6. Avoid extra lookups with nbtree array inequalities.

  7. Enhance nbtree ScalarArrayOp execution.

  8. Improvements and fixes for e0b1ee17dc

  9. Skip checking of scan keys required for directional scan in B-tree

  10. Fix btmarkpos/btrestrpos array key wraparound bug.

  11. Add nbtree high key "continuescan" optimization.

  12. Consider secondary factors during nbtree splits.

  13. Make heap TID a tiebreaker nbtree index column.

  14. Fix planning of btree index scans using ScalarArrayOpExpr quals.

  15. Fix btree stop-at-nulls logic properly.

  16. Teach btree to handle ScalarArrayOpExpr quals natively.

On Sun, Apr 7, 2024 at 8:48 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Coverity pointed out something that looks like a potentially live
> problem in 5bf748b86:
>
> /srv/coverity/git/pgsql-git/postgresql/src/backend/access/nbtree/nbtutils.c: 2950 in _bt_preprocess_keys()
> 2944                              * need to make sure that we don't throw away an array
> 2945                              * scan key.  _bt_compare_scankey_args expects us to
> 2946                              * always keep arrays (and discard non-arrays).
> 2947                              */
> 2948                             Assert(j == (BTEqualStrategyNumber - 1));
> 2949                             Assert(xform[j].skey->sk_flags & SK_SEARCHARRAY);
> >>>     CID 1596256:  Null pointer dereferences  (FORWARD_NULL)
> >>>     Dereferencing null pointer "array".
> 2950                             Assert(xform[j].ikey == array->scan_key);
> 2951                             Assert(!(cur->sk_flags & SK_SEARCHARRAY));
> 2952                         }
> 2953                     }
> 2954                     else if (j == (BTEqualStrategyNumber - 1))
>
> Above this there is an assertion
>
>                     Assert(!array || array->num_elems > 0);
>
> which certainly makes it look like array->scan_key could be
> a null-pointer dereference.

But the "Assert(xform[j].ikey == array->scan_key)" assertion is
located in a block where it's been established that the scan key (the
one stored in xform[j] at this point in execution) must have an array.
It has been marked SK_SEARCHARRAY, and uses the equality strategy, so
it had better have one or we're in big trouble either way.

This is probably very hard for tools like Coverity to understand. We
also rely on the fact that only one of the two scan keys (only one of
the pair of scan keys that were passed to _bt_compare_scankey_args)
can have an array at the point of the assertion that Coverity finds
suspicious. It's possible that both of those scan keys actually did
have arrays, but _bt_compare_scankey_args just treats that as a case
of being unable to prove which scan key was redundant/contradictory
due to a lack of suitable cross-type support -- so the assertion won't
be reached.

Would Coverity stop complaining if I just removed the assertion? I
could just do that, I suppose, but that seems backwards to me.

--
Peter Geoghegan