Summary

There are two key structures in the database that make it possible for Oracle to handle transaction processing and read consistency efficiently: the interested transaction list (ITL) that appears in each data block and lists recent transactions that affected that block, and the transaction table that appears in the segment header block of each undo segment and lists recent transactions that affected the database.

An ITL entry records a transaction ID (xid:), an undo record address (uba:), and a commit SCN. The commit SCN tells Oracle if (and when) the transaction committed. If the commit SCN is not available, then the transaction ID identifies a transaction table slot with sequence number, and this information allows Oracle to check the ...

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