PHOENIX-7924 EXPLAIN improvement bug fixes

[apurtell]

Three fixes for bugs discovered while generating EXPLAIN/EXPLAIN VERBOSE/EXPLAIN JSON examples.

LiteralExpression.toString() rendered typed-null literals as empty. Output now shows BSON_VALUE(..., null) instead of BSON_VALUE(..., ).

Leading whitespace was baked into functional index column names. Fixed by trimming at creation. Output now shows PROJECT "BSON_VALUE(...)" instead of PROJECT " BSON_VALUE(...)".

REWRITE INDEX EXPRESSION <expr> AS <index col> breadcrumb was missing because it was always silently dropped. Drop the eager emission, store the col to expr substitution pairs on StatementContext, and emit one breadcrumb per actually fired substitution from QueryOptimizer.recordDecision.

Co-authored-by: Claude Opus 4.8[1m] noreply@anthropic.com

[Copilot]

Pull request overview

This PR addresses multiple EXPLAIN output correctness issues discovered while generating EXPLAIN/VERBOSE/JSON examples: typed-null literal rendering, functional index column name whitespace, and missing functional-index rewrite breadcrumbs by deferring breadcrumb emission until after the winning plan is chosen.

Changes:

• Fix LiteralExpression.toString() so typed-null literals render as null (avoids empty trailing arguments in function strings).
• Trim top-level functional index expression SQL when creating index column names to avoid leading whitespace being persisted into PColumn names.
• Capture functional-index substitution pairs on StatementContext and emit one rewrite breadcrumb per actually-applied substitution for the chosen plan in QueryOptimizer.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
phoenix-core/src/test/java/org/apache/phoenix/query/explain/ExplainPlanTest.java	Updates expected EXPLAIN JSON/text output for BSON_VALUE to include null for typed-null default argument.
phoenix-core-client/src/main/java/org/apache/phoenix/schema/MetaDataClient.java	Trims serialized functional index expressions before building index column names to prevent leading whitespace in names.
phoenix-core-client/src/main/java/org/apache/phoenix/parse/IndexExpressionParseNodeRewriter.java	Stops eagerly emitting rewrite breadcrumbs during candidate enumeration; retains constructor for compatibility and keeps substitution tracking.
phoenix-core-client/src/main/java/org/apache/phoenix/optimize/QueryOptimizer.java	Records substitution pairs during rewrite and emits rewrite breadcrumbs only for the winner plan.
phoenix-core-client/src/main/java/org/apache/phoenix/expression/LiteralExpression.java	Renders typed-null literals as "null" instead of an empty string.
phoenix-core-client/src/main/java/org/apache/phoenix/compile/StatementContext.java	Adds storage/accessors for applied functional-index substitution pairs to support winner-only breadcrumb emission.

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