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perf(spanner): optimize row popping in StreamedResultSet - #18316
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Eliminates an O(N^2) bottleneck when consuming query results: - Previously, StreamedResultSet yielded rows by calling `iter_rows.pop(0)` in a loop. Because Python lists are contiguous arrays, popping index 0 shifts all remaining elements in memory on every row yielded, causing quadratic overhead for larger result sets. - Replaces `pop(0)` with direct iteration (`for row in iter_rows: yield row`) and detaches the list directly (`iter_rows, self._rows = self._rows, []`), allowing row iteration to run in linear O(N) time at C speed.
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This pull request optimizes row streaming iteration in both the synchronous and asynchronous implementations of StreamedResultSet by replacing an inefficient O(N^2) list-popping loop with a standard O(N) for-loop. It also introduces comprehensive unit tests covering large batches, stepwise consumption, chunk boundaries, early termination, and mid-stream errors. As there are no review comments, I have no feedback to provide.
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(The system test failures are unrelated) |
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🤖 I have created a release *beep* *boop* --- ## [3.72.0](google-cloud-spanner-v3.71.0...google-cloud-spanner-v3.72.0) (2026-11-12) ### Features * **dbapi:** wire timeout parameter through Connection to execute_sql ([#18484](#18484)) ([98fc304](98fc304)) * declare Python3.15 support ([8775794](8775794)) * **spanner:** auth login support for Spanner Omni endpoints ([#18273](#18273)) ([9e0548d](9e0548d)) * update API sources and regenerate ([#18797](#18797)) ([be4d954](be4d954)) ### Bug Fixes * **google-cloud-spanner:** clear buffered DDL statements on rollback and flush on autocommit ([#18636](#18636)) ([15d6a2c](15d6a2c)) * **google-cloud-spanner:** do not replay DDL statements on transaction retry ([#18625](#18625)) ([78b9da2](78b9da2)) * **spanner:** correctly wrap async streaming calls in metrics interceptor ([#18581](#18581)) ([2d19b0a](2d19b0a)) * **spanner:** eliminate duplicate metrics capture and measure full streaming latency ([#18605](#18605)) ([a7678fd](a7678fd)) * **spanner:** record streaming operation completion when stream finishes ([#18704](#18704)) ([ddeae47](ddeae47)) * **spanner:** release transaction lock if inline begin fails ([#18409](#18409)) ([dd24029](dd24029)) * **spanner:** stop deleting multiplexed sessions on rotation and close ([#18332](#18332)) ([c38e960](c38e960)) ### Performance Improvements * **spanner:** add fast-path for multiplexed session acquisition ([#18317](#18317)) ([9569b39](9569b39)) * **spanner:** avoid allocating empty RequestOptions on read and query paths ([#18422](#18422)) ([3743ff0](3743ff0)) * **spanner:** build ExecuteSqlRequest on the raw protobuf message ([#18379](#18379)) ([6d91fec](6d91fec)) * **spanner:** eliminate redundant tracing span in run_in_transaction ([#18357](#18357)) ([181e26a](181e26a)) * **spanner:** optimize built-in metrics hot path and harden concurrency ([#18329](#18329)) ([f2a791f](f2a791f)) * **spanner:** optimize query option merging and prevent in-place mutation ([#18358](#18358)) ([8cb3c6a](8cb3c6a)) * **spanner:** optimize query parameter encoding with direct type dispatch ([#18359](#18359)) ([f06497c](f06497c)) * **spanner:** optimize query result decoding and type resolution ([#18330](#18330)) ([ce72986](ce72986)) * **spanner:** optimize request ID header generation and retry closures ([#18408](#18408)) ([ad03b95](ad03b95)) * **spanner:** optimize row popping in StreamedResultSet ([#18316](#18316)) ([e66103d](e66103d)) * **spanner:** optimize single-chunk results and single-row lookups ([#18602](#18602)) ([05b3775](05b3775)) * **spanner:** prune lock and begin event allocations on single-use snapshots ([#18420](#18420)) ([ab56473](ab56473)) * **spanner:** short-circuit trace_call when tracing is inactive ([#18356](#18356)) ([58031be](58031be)) * **spanner:** support PartialResultSet.last with background stream draining ([#18320](#18320)) ([80efd63](80efd63)) ### Documentation * fix typos ([#18979](#18979)) ([5c8baeb](5c8baeb)) --- This PR was generated with [Release Please](https://github.com/googleapis/release-please). See [documentation](https://github.com/googleapis/release-please#release-please). Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>
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Eliminates an O(N^2) bottleneck when consuming query results:
iter_rows.pop(0)in a loop. Because Python lists are contiguous arrays, popping index 0 shifts all remaining elements in memory on every row yielded, causing quadratic overhead for larger result sets.pop(0)with direct iteration (for row in iter_rows: yield row) and detaches the list directly (iter_rows, self._rows = self._rows, []), allowing row iteration to run in linear O(N) time at C speed.Benchmark Performance Verification
Performance verification of this PR branch (
spanner-row-popping) compared against the 7-day nightly baseline of the Spanner Python client running on GCE (n2-standard-2, 2 vCPUs, closed-loop single worker):Results Summary
spanner-row-popping)main)read-narrow-result-set(200,000 rows, narrow columns)
P50
P90
P99
374.98 ms
474.97 ms
497.50 ms
730.31 ms
2,866.85 ms
2,866.85 ms
-48.65%
-83.43%
-82.65%
~1.9x faster
~6.0x faster
~5.8x faster
read-large-result-set(100,000 rows, wide columns)
P50
P90
P99
2,147.62 ms
2,336.65 ms
2,482.95 ms
2,603.18 ms
2,725.26 ms
2,565.57 ms
-17.50%
-14.26%
-3.22%
~1.21x faster
~1.17x faster
~1.03x faster
Observations
StreamedResultSetwas the dominant CPU bottleneck. Removing that overhead yields a massive performance leap.