fix(planner): Set the size estimate for a ConstantExpression/Literal

[aaneja]

Description, Motivation and Context

The planner generates constant variables which were missing a size estimate. These are needed for correctly estimating join sizes/ join costs (among other cost based comparisons)

Impact

Improves build/prode side estimation during DetermineJoinDistributionType

Test Plan

CI

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Release Notes

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== RELEASE NOTES ==

General Changes
* Improves size estimates for constant variables

Summary by Sourcery

Set size estimates for constant and literal string expressions to improve planner cost estimation for joins and other operations.

Bug Fixes:

• Populate average row size for constant and literal Slice-backed expressions so planner statistics include size estimates.

Tests:

• Extend scalar stats calculator tests to cover average row size estimation for string literals.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Sets average row size estimates for constant scalar expressions backed by Slice values and updates scalar stats tests accordingly, improving join size and cost estimation for constant variables.

Class diagram for updated ScalarStatsCalculator scalar estimation

classDiagram
    class ScalarStatsCalculator {
        +VariableStatsEstimate visitConstant(ConstantExpression literal, Void context)
        +VariableStatsEstimate visitLiteral(Literal node, Void context)
    }

    class VariableStatsEstimate {
        +setLowValue(double lowValue) VariableStatsEstimateBuilder
        +setHighValue(double highValue) VariableStatsEstimateBuilder
        +setAverageRowSize(double averageRowSize) VariableStatsEstimateBuilder
        +build() VariableStatsEstimate
    }

    class VariableStatsEstimateBuilder {
        +setLowValue(double lowValue) VariableStatsEstimateBuilder
        +setHighValue(double highValue) VariableStatsEstimateBuilder
        +setAverageRowSize(double averageRowSize) VariableStatsEstimateBuilder
        +build() VariableStatsEstimate
    }

    class ConstantExpression {
        +Object getValue()
    }

    class Literal {
        +Object getValue()
    }

    class Slice {
        +int length()
    }

    ScalarStatsCalculator --> VariableStatsEstimateBuilder : uses
    ScalarStatsCalculator --> ConstantExpression : parameter
    ScalarStatsCalculator --> Literal : parameter
    ConstantExpression --> Slice : value_may_be
    Literal --> Slice : value_may_be
    VariableStatsEstimateBuilder --> VariableStatsEstimate : builds
    Slice <.. ScalarStatsCalculator : averageRowSize_from_length

Loading

File-Level Changes

Change	Details	Files
Add average row size estimation for constant expressions and literals with Slice values in the scalar stats calculator.	Update ConstantExpression stats calculation to set averageRowSize based on Slice length when the literal value is a Slice. Update Literal stats calculation to set averageRowSize based on Slice length when the value is a Slice.	presto-main-base/src/main/java/com/facebook/presto/cost/ScalarStatsCalculator.java
Extend scalar stats calculator tests to cover average row size estimation for string literals.	Capture the VariableStatsAssertion for a simple StringLiteral and assert that averageRowSize matches the literal length. Keep existing assertions for distinct values, low/high values, and null fraction for the StringLiteral.	presto-main-base/src/test/java/com/facebook/presto/cost/TestScalarStatsCalculator.java

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[sourcery-ai]

Hey - I've left some high level feedback:

• The logic to set averageRowSize for Slice values is duplicated in both visitConstant and visitLiteral; consider extracting a small helper to keep this behavior consistent and easier to evolve.
• Right now only Slice-backed literals get an averageRowSize; if there are other variable-width types represented differently (e.g., non-Slice-backed), it may be worth clarifying or centralizing how average row size should be derived across all relevant types.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The logic to set `averageRowSize` for `Slice` values is duplicated in both `visitConstant` and `visitLiteral`; consider extracting a small helper to keep this behavior consistent and easier to evolve.
- Right now only `Slice`-backed literals get an `averageRowSize`; if there are other variable-width types represented differently (e.g., non-Slice-backed), it may be worth clarifying or centralizing how average row size should be derived across all relevant types.

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