perf(planner): Skip unnecessary processing for leaf expressions and trivial predicates in wide projections

[feilong-liu]

Description

Three targeted fast-path optimizations for wide-column queries (e.g., FAB BA capping query with ~2225 column assignments):

1. SymbolMapper.map(RowExpression): Skip TreeRewriter instantiation for leaf expressions (VariableReferenceExpression and ConstantExpression). For identity assignments, call map(VariableReferenceExpression) directly instead of creating an anonymous RowExpressionRewriter + TreeRewriter.

2. PruneUnreferencedOutputs.visitProject: Short-circuit VariablesExtractor.extractUnique() for VariableReferenceExpression assignments by directly adding the variable, avoiding ImmutableList.Builder, visitor traversal, and ImmutableSet allocation.

3. PredicatePushDown.visitProject: Early return when inherited predicate is TRUE_CONSTANT (no conjuncts to push), skipping isDeterministic evaluation on all assignments.

Motivation and Context

For wide projections with ~2225 assignments, the majority are passthrough identity variables (col := col). Each of these was going through expensive processing paths that produce no useful work:

• SymbolMapper instantiated a full RowExpressionTreeRewriter + anonymous RowExpressionRewriter for every leaf expression
• PruneUnreferencedOutputs ran full VariablesExtractor.extractUnique() with visitor traversal for simple variable references
• PredicatePushDown evaluated isDeterministic on all assignments even when the inherited predicate is trivially TRUE

Impact

Reduces optimizer overhead for queries with wide projections. No functional behavior changes — all optimizations are pure fast paths that produce identical results.

Test Plan

• Added testMapRowExpressionVariable, testMapRowExpressionConstant, testMapRowExpressionUnmappedVariable in TestSymbolMapper
• Added testProjectWithVariableReferenceAssignments in TestPruneUnreferencedOutputs
• Added testNoPredicateOverProjection in TestPredicatePushdown

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Release Notes

== RELEASE NOTES ==
General Changes
* Improve query planning performance for wide projections by skipping unnecessary processing for leaf expressions and trivial predicates in SymbolMapper, PruneUnreferencedOutputs, and PredicatePushDown.

[sourcery-ai]

Reviewer's Guide

Introduces targeted fast-path optimizations in the query planner for wide-column project nodes by short‑circuiting common cases in symbol mapping, pruning unreferenced outputs, and predicate pushdown without changing semantics.

File-Level Changes

Change	Details	Files
Add fast-path handling for leaf RowExpressions in SymbolMapper.map to avoid unnecessary TreeRewriter usage.	Return mapped VariableReferenceExpression directly when the input RowExpression is a VariableReferenceExpression. Return ConstantExpression as-is without invoking RowExpressionTreeRewriter. Retain existing RowExpressionTreeRewriter path for non-leaf expressions.	presto-main-base/src/main/java/com/facebook/presto/sql/planner/optimizations/SymbolMapper.java
Optimize PruneUnreferencedOutputs project visitation by avoiding full variable extraction for simple variable assignments.	Detect VariableReferenceExpression assignments and add them directly to expectedInputs. Fallback to VariablesExtractor.extractUnique for non-variable expressions. Keep existing logic for building the filtered Assignments based on referenced variables.	presto-main-base/src/main/java/com/facebook/presto/sql/planner/optimizations/PruneUnreferencedOutputs.java
Add an early-return fast path in PredicatePushDown when the inherited predicate is TRUE to skip determinism checks.	Check for TRUE_CONSTANT inherited predicate at the start of visitProject. When TRUE_CONSTANT, call context.defaultRewrite on the ProjectNode with TRUE_CONSTANT and skip determinism evaluation over assignments. Preserve existing deterministic variable collection and predicate pushdown logic for non-TRUE inherited predicates.	presto-main-base/src/main/java/com/facebook/presto/sql/planner/optimizations/PredicatePushDown.java

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[sourcery-ai]

Hey - I've left some high level feedback:

• The fast path in PredicatePushDown.visitProject should likely check context.get() == TRUE_CONSTANT rather than .equals(TRUE_CONSTANT), since an arbitrary expression that evaluates to true but still contains conjuncts would now bypass predicate pushdown, changing behavior rather than just optimizing.
• In the same fast path, consider early-returning using the existing context.get() instead of hardcoding TRUE_CONSTANT into defaultRewrite to avoid subtly changing behavior if a non-literal true predicate ever reaches this code path.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The fast path in `PredicatePushDown.visitProject` should likely check `context.get() == TRUE_CONSTANT` rather than `.equals(TRUE_CONSTANT)`, since an arbitrary expression that evaluates to true but still contains conjuncts would now bypass predicate pushdown, changing behavior rather than just optimizing.
- In the same fast path, consider early-returning using the existing `context.get()` instead of hardcoding `TRUE_CONSTANT` into `defaultRewrite` to avoid subtly changing behavior if a non-literal true predicate ever reaches this code path.

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