perf: Optimize SQL string construction

[emicklei]

The SQL() function in sql_writing.go was inefficiently building the SQL string. It used an intermediate formatted string created with a bytes.Buffer, and then performed multiple strings.ReplaceAll calls to clean it up.

This change introduces a custom io.Writer implementation, onelineWriter, that builds the final, single-line SQL string in a single pass. It replaces tabs and newlines with spaces and collapses multiple spaces into one on the fly.

This new approach reduces memory allocations and CPU usage, improving the performance of every query built by the library.

[Copilot]

Pull Request Overview

This PR optimizes SQL string construction by replacing inefficient string manipulation with a custom writer implementation. The optimization improves performance by reducing memory allocations and CPU usage when building SQL queries.

Key changes:

• Introduces onelineWriter that formats SQL strings in a single pass instead of multiple string replacements
• Optimizes result iteration by pre-ordering column selectors to avoid nested loops
• Refactors iterator logic to use ordered selectors for improved performance

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
sql_writing.go	Implements onelineWriter and refactors SQL() function for single-pass formatting
queryset.go	Pre-orders selectors using a map lookup to optimize result iteration
mutationset.go	Applies similar selector ordering optimization for mutation results
iterator.go	Updates to use pre-ordered selectors, eliminating nested loops in Next()

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