Fuse multiple scalar-aggregate subqueries over the same source into a single scan

[nathanb9]

Which issue does this PR close?

• Closes Fuse multiple scalar-aggregate subqueries over the same source into a single scan #23213.

Rationale for this change

When a query computes several uncorrelated scalar-aggregate subqueries over the same source, DataFusion scans that source once per subquery. When the subqueries share an identical source, they can be computed in a single scan by pushing each predicate into a FILTER (WHERE ...) clause.

What changes are included in this PR?

A new logical optimizer rule, FuseScalarSubqueries, gated by datafusion.optimizer.enable_fuse_scalar_subqueries (default off). When a projection contains 2 or more uncorrelated scalar-aggregate subqueries over a structurally identical source, the rule fuses them into a single aggregate:

-- Before: two scans of t
SELECT (SELECT count(*) FROM t WHERE a < 10),
       (SELECT avg(x)   FROM t WHERE a >= 10);

-- After: one scan of t
SELECT count(*) FILTER (WHERE a < 10),
       avg(x)   FILTER (WHERE a >= 10)
FROM t;

The source filter becomes the OR of the branch predicates, and each scalar subquery is replaced by a reference to the merged aggregate column. The rule runs before subquery decorrelation and is conservative: it skips correlated, DISTINCT, ordered, or volatile aggregates, and predicates containing subqueries.

This is an initial, opt-in version mirroring the existing enable_unions_to_filter rule. Follow-ups could add a size or selectivity guard.

[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-common v54.0.0 (current)
       Built [  35.344s] (current)
     Parsing datafusion-common v54.0.0 (current)
      Parsed [   0.061s] (current)
    Building datafusion-common v54.0.0 (baseline)
       Built [  33.330s] (baseline)
     Parsing datafusion-common v54.0.0 (baseline)
      Parsed [   0.060s] (baseline)
    Checking datafusion-common v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.659s] 223 checks: 222 pass, 1 fail, 0 warn, 30 skip

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field OptimizerOptions.enable_fuse_scalar_subqueries in /home/runner/work/datafusion/datafusion/datafusion/common/src/config.rs:1347

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  70.552s] datafusion-common
    Building datafusion-optimizer v54.0.0 (current)
       Built [  27.413s] (current)
     Parsing datafusion-optimizer v54.0.0 (current)
      Parsed [   0.032s] (current)
    Building datafusion-optimizer v54.0.0 (baseline)
       Built [  26.890s] (baseline)
     Parsing datafusion-optimizer v54.0.0 (baseline)
      Parsed [   0.030s] (baseline)
    Checking datafusion-optimizer v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.161s] 223 checks: 223 pass, 30 skip
     Summary no semver update required
    Finished [  55.274s] datafusion-optimizer
    Building datafusion-sqllogictest v54.0.0 (current)
       Built [ 177.812s] (current)
     Parsing datafusion-sqllogictest v54.0.0 (current)
      Parsed [   0.022s] (current)
    Building datafusion-sqllogictest v54.0.0 (baseline)
       Built [ 180.902s] (baseline)
     Parsing datafusion-sqllogictest v54.0.0 (baseline)
      Parsed [   0.022s] (baseline)
    Checking datafusion-sqllogictest v54.0.0 -> v54.0.0 (no change; assume patch)
     Checked [   0.091s] 223 checks: 223 pass, 30 skip
     Summary no semver update required
    Finished [ 361.368s] datafusion-sqllogictest