fix string partitioning edge case to avoid rabbit-holing into single-row partitions

[jgao54]

Follow up to #4522

Main thing to clean up is the edge case where if both start and end boundary string's first differing byte (the most significant byte for determining the midpoint) are below or above printable ascii, continuing with midpoint interpolation in the extreme case can result in generating a lot of single-row partitions, until we are left with one large final partition. Terminate early when this happens to avoid introducing unnecessary latency. It may still be a full table snapshot, but we don't want to generate a lot of meaningless partitions.

[jgao54]

wrapped this up into test above; since we are now checking ascii boundaries, most of these cases would no longer be hit, as they would be deemed unsplittable before then.

[github-actions]

🔄 Flaky Test Detected

Analysis: The flow/e2e suite hit the 20-minute package timeout because an unrelated test (TestApiPg/TestDropMissing) stalled for 5m34s, and only one matrix combo (mysql-gtid) failed while all others passed on the same commit — indicating an environmental/flaky timeout rather than a real bug.
Confidence: 0.85

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[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
3147	2	3145	415

View the top 2 failed test(s) by shortest run time

github.com/PeerDB-io/peerdb/flow/e2e::TestGenericBQ

Stack Traces | 0.01s run time

=== RUN   TestGenericBQ
=== PAUSE TestGenericBQ
=== CONT  TestGenericBQ
--- FAIL: TestGenericBQ (0.01s)

github.com/PeerDB-io/peerdb/flow/e2e::TestGenericBQ/Test_Simple_Flow

Stack Traces | 63.3s run time

=== RUN   TestGenericBQ/Test_Simple_Flow
=== PAUSE TestGenericBQ/Test_Simple_Flow
=== CONT  TestGenericBQ/Test_Simple_Flow
2026/07/24 21:25:17 INFO Executing and processing query x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf\" ORDER BY id"
2026/07/24 21:25:17 INFO Executing and processing query stream x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf\" ORDER BY id"
2026/07/24 21:25:17 INFO [pg_query_executor] declared cursor x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursorQuery="DECLARE peerdb_cursor_9700098224048205552 CURSOR FOR SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf\" ORDER BY id" args=[]
2026/07/24 21:25:17 INFO [pg_query_executor] fetching rows start x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf\" ORDER BY id" channelLen=0
2026/07/24 21:25:17 INFO [pg_query_executor] fetching from cursor x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_9700098224048205552
2026/07/24 21:25:17 INFO processed row stream x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_9700098224048205552 records=8 bytes=168 channelLen=7
2026/07/24 21:25:17 INFO [pg_query_executor] fetched rows x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf\" ORDER BY id" rows=8 bytes=168 channelLen=7
2026/07/24 21:25:17 INFO [pg_query_executor] fetching from cursor x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_9700098224048205552
2026/07/24 21:25:17 INFO processed row stream x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_9700098224048205552 records=0 bytes=0 channelLen=0
2026/07/24 21:25:17 INFO [pg_query_executor] fetched rows x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf\" ORDER BY id" rows=0 bytes=0 channelLen=0
2026/07/24 21:25:17 INFO [pg_query_executor] committing transaction x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart
2026/07/24 21:25:17 INFO [pg_query_executor] committed transaction for query x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf\" ORDER BY id" rows=8 bytes=168 channelLen=0
2026/07/24 21:25:17 INFO Executing and processing query x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf_dst\" ORDER BY id"
2026/07/24 21:25:17 INFO Executing and processing query stream x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf_dst\" ORDER BY id"
2026/07/24 21:25:17 INFO [pg_query_executor] declared cursor x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursorQuery="DECLARE peerdb_cursor_15961657269615954898 CURSOR FOR SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf_dst\" ORDER BY id" args=[]
2026/07/24 21:25:17 INFO [pg_query_executor] fetching rows start x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf_dst\" ORDER BY id" channelLen=0
2026/07/24 21:25:17 INFO [pg_query_executor] fetching from cursor x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_15961657269615954898
2026/07/24 21:25:17 INFO processed row stream x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_15961657269615954898 records=3 bytes=63 channelLen=2
2026/07/24 21:25:17 INFO [pg_query_executor] fetched rows x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf_dst\" ORDER BY id" rows=3 bytes=63 channelLen=2
2026/07/24 21:25:17 INFO [pg_query_executor] fetching from cursor x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_15961657269615954898
2026/07/24 21:25:17 INFO processed row stream x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart cursor=peerdb_cursor_15961657269615954898 records=0 bytes=0 channelLen=0
2026/07/24 21:25:17 INFO [pg_query_executor] fetched rows x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf_dst\" ORDER BY id" rows=0 bytes=0 channelLen=0
2026/07/24 21:25:17 INFO [pg_query_executor] committing transaction x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart
2026/07/24 21:25:17 INFO [pg_query_executor] committed transaction for query x-peerdb-additional-metadata={Operation:FLOW_OPERATION_UNKNOWN} partitionId=testpart query="SELECT id,name,created_at FROM e2e_test_pg_rllmvw7x.\"test_inheritance_dynconf_dst\" ORDER BY id" rows=3 bytes=63 channelLen=0
    generic_test.go:143: UNEXPECTED STATUS TIMEOUT STATUS_SNAPSHOT
    bigquery.go:86: begin tearing down postgres schema bq_4tdduqmn_20260724212517
--- FAIL: TestGenericBQ/Test_Simple_Flow (63.28s)

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[github-actions]

🔄 Flaky Test Detected

Analysis: Both failed jobs timed out identically waiting for BigQuery's initial snapshot (TestGenericBQ/Test_Simple_Flow, "UNEXPECTED STATUS TIMEOUT STATUS_SNAPSHOT") — a timeout in an external-service e2e test unrelated to the PR's QRep string-partitioning change, indicating transient BigQuery slowness rather than a real bug.
Confidence: 0.85

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