Re: Adding skip scan (including MDAM style range skip scan) to nbtree

Hi Peter,

> I'm now very close to committing everything. Though I do still want
> another pair of eyes on the newer
> 0003-Improve-skip-scan-primitive-scan-scheduling.patch stuff before
> commiting (since I still intend to commit all the remaining patches
> together).

Can you think of any tests specifically for 0003, or relying on the
added Asserts() is best we can do? Same question for 0002.

I can confirm that v33 applies and passes the test.

0002 adds _bt_set_startikey() to nbtutils.c but it is not well-covered
by tests, many branches of the new code are never executed.

```
@@ -2554,9 +2865,20 @@ _bt_check_compare(IndexScanDesc scan, ScanDirection dir,
*/
if (requiredSameDir)
*continuescan = false;
+ else if (unlikely(key->sk_flags & SK_BT_SKIP))
+ {
+ /*
+ * If we're treating scan keys as nonrequired, and encounter a
+ * skip array scan key whose current element is NULL, then it
+ * must be a non-range skip array
+ */
+ Assert(forcenonrequired && *ikey > 0);
+ return _bt_advance_array_keys(scan, NULL, tuple, tupnatts,
+ tupdesc, *ikey, false);
+ }
```

This branch is also never executed during the test run.

In 0003:

```
@@ -2006,6 +2008,10 @@ _bt_advance_array_keys(IndexScanDesc scan,
BTReadPageState *pstate,
else if (has_required_opposite_direction_only && pstate->finaltup &&
unlikely(!_bt_oppodir_checkkeys(scan, dir, pstate->finaltup)))
{
+ /*
+ * Make sure that any SAOP arrays that were not marked required by
+ * preprocessing are reset to their first element for this direction
+ */
_bt_rewind_nonrequired_arrays(scan, dir);
goto new_prim_scan;
}
```

This branch is never executed too. This being said, technically there
is no new code here.

For your convenience I uploaded a complete HTML code coverage report
(~36 Mb) [1].

[1]: https://drive.google.com/file/d/1breVpHapvJLtw8AlFAdXDAbK8ZZytY6v/view?usp=sharing

--
Best regards,
Aleksander Alekseev