Use sync.Pool for mergeIterator batchesBuf - #7765
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Replace per-iterator batchesBuf with a sync.Pool. Each buildNextBatch call borrows a scratch buffer from the pool, merges into it, copies the result into c.batches, and returns the buffer immediately. Idle iterators hold no buffer at all. This reduces heap usage from O(series) to O(concurrent_evaluations) — saving ~1200 bytes per series (~11 GB at 9M series). The pool approach is engine-agnostic: safe for both sequential and parallel series processing within a single Select call. Signed-off-by: Paurush Garg <paurushg@amazon.com>
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What this PR does:
Each
mergeIteratorallocates its ownbatchesBufat creation time. This adds to the heap profile for query engines that create all iterators upfront, especially with high-cardinality queries - while only one buffer is actively used at a time since series are processed sequentially.This PR replaces per-iterator allocation with a sync.Pool. Each
buildNextBatchcall borrows a scratch buffer, merges into it, copies the result into c.batches, and returns the buffer via defer. Idle iterators hold no buffer.Checklist
CHANGELOG.mdupdated - the order of entries should be[CHANGE],[FEATURE],[ENHANCEMENT],[BUGFIX]docs/configuration/v1-guarantees.mdupdated if this PR introduces experimental flags