Allow merges for single dataformat

[alchemist51]

Description

[Describe what this change achieves]

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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PR Reviewer Guide 🔍

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🧪 No relevant tests

🔒 No security concerns identified

📝 TODO sections server/src/main/java/org/opensearch/index/engine/exec/merge/CompositeMergeHandler.java [50]: Remove this once the Merge is working for multi format setup

🔀 No multiple PR themes

⚡ Recommended focus areas for review TODO Incomplete The multi-format merge path uses NoMergePolicy.INSTANCE as a temporary workaround with a TODO comment indicating it needs to be replaced. This means merges are completely disabled for multi-format setups, which could lead to unbounded segment growth and performance degradation in production if this code path is reached. } else { // TODO:: Remove this once the Merge is working for multi format setup mergePolicy = new CompositeMergePolicy(NoMergePolicy.INSTANCE, shardId); } Missing Case Handling The inner switch statement for nodeMergePolicy in the DEFAULT_POLICY branch does not have a default case. If a new IndexMergePolicy enum value is added in the future, mergePolicyProvider could remain null, causing the assertion to fire at runtime. Consider adding a default case with an explicit error or fallback. switch (nodeMergePolicy) { case TIERED: case DEFAULT_POLICY: mergePolicyProvider = tieredMergePolicyProvider; break; case LOG_BYTE_SIZE: mergePolicyProvider = logByteSizeMergePolicyProvider; break; }

[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
Possible issue	Add default case to prevent null provider The inner switch on nodeMergePolicy has no default case, so if a new IndexMergePolicy enum value is added in the future, mergePolicyProvider will remain null and the subsequent assert will fail at runtime. Adding a default case that throws an IllegalStateException makes the failure explicit and easier to diagnose. server/src/main/java/org/opensearch/index/IndexSettings.java [1945-1953] switch (nodeMergePolicy) { case TIERED: case DEFAULT_POLICY: mergePolicyProvider = tieredMergePolicyProvider; break; case LOG_BYTE_SIZE: mergePolicyProvider = logByteSizeMergePolicyProvider; break; + default: + throw new IllegalStateException("Unsupported node-scoped merge policy: " + nodeMergePolicy); } Suggestion importance[1-10]: 6 __ Why: Adding a default case with an explicit exception is a valid defensive programming improvement. Without it, adding a new IndexMergePolicy enum value could leave mergePolicyProvider null, causing the assert to fail with a less informative message. However, the outer switch has the same issue and the assert already provides some safety net.	Low
General	Warn when merging is silently disabled The NoMergePolicy.INSTANCE path is a temporary workaround that silently disables merging for multi-format setups. This could lead to unbounded segment growth and degraded query performance in production if the TODO is not addressed. Consider throwing an explicit exception or logging a prominent warning to prevent accidental use in production environments. server/src/main/java/org/opensearch/index/engine/exec/merge/CompositeMergeHandler.java [46-52] List<String> secondaryDataFormatNames = indexSettings.getValue(IndexSettings.INDEX_COMPOSITE_SECONDARY_DATA_FORMATS_SETTING); if(secondaryDataFormatNames.isEmpty()) { mergePolicy = new CompositeMergePolicy(indexSettings.getMergePolicy(true), shardId); } else { // TODO:: Remove this once the Merge is working for multi format setup + logger.warn("Merging is disabled for multi-format composite index setup on shard [{}]. Segments will not be merged.", shardId); mergePolicy = new CompositeMergePolicy(NoMergePolicy.INSTANCE, shardId); } Suggestion importance[1-10]: 5 __ Why: Adding a warning log when NoMergePolicy.INSTANCE is used is a reasonable improvement to make the temporary workaround more visible, but it's a minor enhancement since the TODO comment already documents the intent and the code path is intentional.	Low

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❌ Gradle check result for c0d0236: null

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?