Add support for RETURNING clause in bulk insert queries

[tomtwinkle]

Overview

Fixed support for PostgreSQL's RETURNING clause in bulk insert queries. The enhancement ensures that bulk insert query generation correctly preserves the RETURNING clause when transforming single-row insert statements into multi-row batch inserts.

Changes

• Modified templates/template.go: Added logic to detect and preserve the RETURNING keyword as a suffix clause (similar to existing handling for ON DUPLICATE KEY UPDATE and ON CONFLICT)
• Added templates/template_test.go: Introduced 4 new test cases covering:
  • Basic RETURNING clause functionality
  • RETURNING clause combined with ON CONFLICT (PostgreSQL upsert pattern)
  • Additional test coverage for ON DUPLICATE KEY UPDATE (MySQL upsert) including case-insensitive handling

Impact

• PostgreSQL compatibility: Users can now generate bulk insert queries that return inserted row data (e.g., auto-generated IDs)
• Database feature parity: Brings RETURNING clause support in line with existing upsert clause handling
• No breaking changes: The modification extends existing suffix detection logic without affecting current functionality
• Improved test coverage: 52 new lines of test code ensure the feature works correctly in isolation and combined with other SQL clauses

[Copilot]

Pull Request Overview

This PR adds support for PostgreSQL's RETURNING clause in bulk insert queries by extending the suffix detection logic to recognize and preserve RETURNING alongside existing upsert clauses (ON DUPLICATE KEY UPDATE and ON CONFLICT).

Key changes:

• Enhanced suffix detection in buildBulkInsertQuery to identify the earliest occurrence of suffix keywords (ON DUPLICATE KEY UPDATE, ON CONFLICT, RETURNING)
• Refactored query parsing to extract suffixes before searching for the VALUES clause, preventing false matches within suffix functions
• Added comprehensive test coverage for RETURNING clause (standalone and combined with ON CONFLICT), plus additional upsert test cases

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
templates/template.go	Refactored suffix detection logic to support RETURNING clause alongside existing upsert keywords; simplified iteration syntax
templates/template_test.go	Added 8 new test cases covering RETURNING clause, upsert patterns, and case-insensitive handling

Comments suppressed due to low confidence (1)

templates/template.go:103

• The queryPrefixStr should use queryWithoutSuffix instead of trimmedQuery to avoid including the suffix in the prefix. If a suffix exists, the current code will include it in both the prefix and suffix parts of the final query.

	queryPrefixStr := strings.TrimSpace(trimmedQuery[:valuesUpperIndex]) + " VALUES "

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