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When created, a stream logically takes an initial snapshot of every row in the source object (e.g. table, external table, or the underlying tables for a view) by initializing a point in time (called an offset) as the current transactional version of the object. The change tracking system utilized by the stream then records information about the DML changes after this snapshot was taken. Change records provide the state of a row before and after the change. Change information mirrors the column structure of the tracked source object and includes additional metadata columns that describe each change event. Note that a stream itself does not contain any table data. A stream only stores an offset for the source object and returns CDC records by leveraging the versioning history for the source object. A new table version is created whenever a transaction that includes one or more DML statements is committed to the table. In the transaction history for a table, a stream offset is located between two table versions. Querying a stream returns the changes caused by transactions committed after the offset and at or before the current time. Multiple queries can independently consume the same change data from a stream without changing the offset. A stream advances the offset only when it is used in a DML transaction. This behavior applies to both explicit and autocommit transactions. (By default, when a DML statement is execut-ed, an autocommit transaction is implicitly started and the transaction is committed at the completion of the statement. This behavior is controlled with the AUTOCOMMIT parameter.) Querying a stream alone does not advance its offset, even within an explicit transaction; the stream contents must be consumed in a DML statement. To advance the offset of a stream to the current table version without consuming the change data in a DML operation, complete either of the following actions:· Recreate the stream (using the CREATE OR REPLACE STREAM syntax). Insert the current change data into a temporary table. In the INSERT statement, query the stream but include a WHERE clause that filters out all of the change data (e.g. WHERE 0 = 1). 

Streams support repeatable read isolation. In repeatable read mode, multiple SQL statements within a transaction see the same set of records in a stream. This differs from the read committed mode supported for tables, in which statements see any changes made by previous statements executed within the same transaction, even though those changes are not yet committed. The delta records returned by streams in a transaction is the range from the current position of the stream until the transaction start time. The stream position advances to the transaction start time if the transaction commits; otherwise, it stays at the same position. Within Transaction 1, all queries to stream s1 see the same set of records. DML changes to table t1 are recorded to the stream only when the transaction is committed. In Transaction 2, queries to the stream see the changes recorded to the table in Transaction 1. Note that if Transaction 2 had begun before Transaction 1 was committed, queries to the stream would have returned a snapshot of the stream from the position of the stream to the beginning time of Transaction 2 and would not see any changes committed by Transaction 1.

Stream Columns A stream stores an offset for the source object and not any actual table columns or data. When queried, a stream accesses and returns the historic data in the same shape as the source object (i.e. the same column names and ordering) with the following additional columns:METADATA$ACTION Indicates the DML operation (INSERT, DELETE) recorded. METADATA^^SUPDATE Indicates whether the operation was part of an UPDATE statement. Updates to rows in the source object are represented as a pair of DELETE and INSERT records in the stream with a metadata column METADATA^^SUPDATE values set to TRUE. METADATA$ROW_ID Specifies the unique and immutable ID for the row, which can be used to track changes to specific rows over time. Note that streams record the differences between two offsets. If a row is added and then updated in the current offset, the delta change is a new row. The METADATA^^SUPDATE row records a FALSE value.