bench: add projection_subquery SQL benchmark suite

[adriangb]

Which issue does this PR close?

• This PR closes no issue. It is related to Projected IN subqueries plan two extra nested-loop mark joins per subquery and become quadratic #25341 and it prepares the A/B measurement for perf: use one null-aware mark join for hashable IN subqueries in projections #25338.

Rationale for this change

An IN, NOT IN or EXISTS subquery in the SELECT list is much slower than the same subquery in a WHERE clause.
A projected IN must return NULL, and not false, when there is no match and the inner side holds a NULL.
The decorrelation therefore turns one projected IN into more than one mark join.
A mark join with no hashable join predicate runs as a nested-loop join, so the cost grows with the outer row count times the inner row count.
There is no benchmark suite that measures this shape today.

The benchmark bot compares a pull request against its merge base.
It can only report a change if the suite is present on both sides.
The suite must therefore land on main first, before the fix is measured with it.

What changes are included in this PR?

• A new declarative SQL benchmark suite, projection_subquery, with seven queries. Each query is one shape:
  • q01_in_bare: bare uncorrelated IN.
  • q02_in_coalesce: the same IN wrapped in COALESCE, which is the common way to use the result.
  • q03_in_correlated_eq: IN correlated on an equality.
  • q04_in_two_columns: two independent IN subqueries in one SELECT list.
  • q05_not_in_bare: NOT IN.
  • q06_exists_correlated: correlated EXISTS, which has no NULL result and needs only one mark join.
  • q07_in_correlated_residual: IN correlated on <. There is no equality to hash on, so this query keeps the nested-loop plan. It is the control: its time must not change when the hashable shapes get faster.
• Two small integer tables built inline by the suite load SQL, so the suite needs no data step. Every 97th inner key is NULL, which keeps three-valued logic in play. Set PSQ_ROWS to change the row count in each table. The default is 30000.
• An aggregate over the projected boolean in every query, so the result is two numbers and the measured cost is the plan and not the size of the output.
• Checked-in result files, so --result-mode validate also proves that a change to the decorrelation still returns the same rows. The counts hold for the default PSQ_ROWS.
• bench.sh wiring: ./benchmarks/bench.sh data projection_subquery reports that there is no external data, and ./benchmarks/bench.sh run projection_subquery runs the suite.
• Documentation in benchmarks/README.md and in the suite table of benchmarks/sql_benchmarks/README.md.

What is the testing strategy for this PR?

This PR adds no product code, so there are no unit tests. It was verified by running the suite.

./benchmarks/bench.sh data projection_subquery prints the no-data message.
./benchmarks/bench.sh run projection_subquery builds cargo bench --bench sql and completes with a time for each of the seven queries.
--result-mode validate passes on main and on the branch of #25338, which shows that both sides return the same rows.

Median of five iterations at the default PSQ_ROWS of 30000, on one machine:

Query	main	With #25338
q01_in_bare	341 ms	0.8 ms
q02_in_coalesce	690 ms	1.1 ms
q03_in_correlated_eq	2.6 ms	0.9 ms
q04_in_two_columns	527 ms	1.2 ms
q05_not_in_bare	349 ms	0.6 ms
q06_exists_correlated	0.6 ms	0.4 ms
q07_in_correlated_residual (control)	66 ms	63 ms

The control query is unchanged, as expected, because it has no equality to hash on.

Are there any user-facing changes?

No. This PR adds benchmarks only.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.33%. Comparing base (edc936f) to head (85da22e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25346      +/-   ##
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- Coverage   82.33%   82.33%   -0.01%     
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  Files        1137     1137              
  Lines      432498   432498              
  Branches   432498   432498              
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- Hits       356116   356107       -9     
- Misses      54843    54851       +8     
- Partials    21539    21540       +1     

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