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Editorial Team
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Asked: 2026-05-24T17:24:38+00:00

Imagine I have this simple table: Table Name: Table1 Columns: Col1 NUMBER (Primary Key)

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Imagine I have this simple table:

Table Name: Table1
Columns:    Col1 NUMBER (Primary Key)
            Col2 NUMBER

If I insert a record into Table1 with no commit…

INSERT INTO Table1 (Col1, Col2) Values (100, 1234);

How does Oracle know that this next INSERT statement violates the PK constraint, since nothing has yet been committed to the database yet.

INSERT INTO Table1 (Col1, Col2) Values (100, 5678);

Where/how does Oracle manage the transactions so that it knows I’m violating the constraint when I haven’t even committed the transaction yet.

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1 Answer

  1. Editorial Team

    Oracle creates an index to enforce the primary key constraint (a unique index by default). When Session A inserts the first row, the index structure is updated but the change is not committed. When Session B tries to insert the second row, the index maintenance operation notes that there is already a pending entry in the index with that particular key. Session B cannot acquire the latch that protects the shared index structure so it will block until Session A’s transaction completes. At that point, Session B will either be able to acquire the latch and make its own modification to the index (because A rolled back) or it will note that the other entry has been committed and will throw a unique constraint violation (because A committed).

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