![]() ![]() The third item in the previous list is a more insidious cause of ORA-01555 in that it can happen in a database where there is a single session, and this session is not modifying the tables that are being queried when the ORA-01555 error is raised! This doesn t seem possible why would we need undo data for a table we can guarantee is not being modified We ll find out shortly. It is this dual use of the undo segments, both to roll back failed transactions and to provide for read consistency that results in the ORA-01555 error. The INSERT reads the table to find rows where X=2, and then INSERTs them, and so on. In the example just shown, the UPDATE reads the table to find rows where x=2 (and then UPDATEs them). All statements that read the table take advantage of this read consistency. ![]()
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