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MetaDave ๐Ÿ‡ช๐Ÿ‡บ

Very much RDBMS dependent in this respect, and I appreciate that this article and the responses focus on SQL Server.

In Oracle a CTE can be persisted by the query optimiser, and the decision is likely to be influenced by its expected size. The data is written to a temporary tablespace. Of course it is also unindexed.