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Check transaction applied after ingestion

documentation/cookbook/operations/check-transaction-applied.md

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When ingesting data to a WAL table using ILP protocol, inserts are asynchronous. This recipe shows how to ensure all ingested rows are visible for read-only queries.

Problem

You're performing a single-time ingestion of a large data volume using ILP protocol to a table that uses Write-Ahead Log (WAL). Since inserts are asynchronous, you need to confirm that all ingested rows are visible for read-only queries before proceeding with operations.

Solution

Query the wal_tables() function to check if the writer transaction matches the sequencer transaction. When these values match, all rows have become visible:

questdb-sql
SELECT *
FROM wal_tables()
WHERE name = 'core_price' AND writerTxn = sequencerTxn;

This query returns a row when writerTxn equals sequencerTxn for your table:

  • writerTxn is the last committed transaction available for read-only queries
  • sequencerTxn is the last transaction committed to WAL

When they match, all WAL transactions have been applied and all rows are visible for queries.

Another viable approach is to run SELECT count(*) FROM my_table and verify the expected row count.

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