Details
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Bug
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Status: Resolved
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P2
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Resolution: Fixed
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None
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None
Description
https://stackoverflow.com/questions/48784889/streaming-data-from-cloudsql-into-dataflow/48819934#48819934 - a user is trying to load a large table from MySQL, and the MySQL JDBC driver requires special measures when loading large result sets.
JdbcIO currently calls simply "connection.prepareStatement(query)" https://github.com/apache/beam/blob/bb8c12c4956cbe3c6f2e57113e7c0ce2a5c05009/sdks/java/io/jdbc/src/main/java/org/apache/beam/sdk/io/jdbc/JdbcIO.java#L508 - it should specify type TYPE_FORWARD_ONLY and concurrency CONCUR_READ_ONLY - these values should always be used.
Seems that different databases have different requirements for streaming result sets.
E.g. MySQL requires setting fetch size; PostgreSQL says "The Connection must not be in autocommit mode." https://jdbc.postgresql.org/documentation/head/query.html#query-with-cursor . Oracle, I think, doesn't have any special requirements but I don't know. Fetch size should probably still be set to a reasonably large value.
Seems that the common denominator of these requirements is: set fetch size to a reasonably large but not maximum value; disable autocommit (there's nothing to commit in read() anyway).
