Chained Row Ratio

Chained rows occurs as a result of row migration or row chaining.

Row chaining When the size of a row is more than the size of the data block, then the row is spread across multiple blocks. This is called Row Chaining.

Row migration When an update increases the row size and there is no free space available in the data block to accommodate the row, the entire row will be moved to a new block leaving only a pointer in the original block. A row migration can also lead to a row chain if the row becomes large enough.

Row chaining affects index reads and full table scans. You cannot do much to avoid row chaining, one way is to calculate the length of the row and put it in a tablespace that has large block size.

Row migration does not affect full table scans but affects index reads. Increase the PCTFREE to allow more space for future updates. Re-organize or recreate the tables and indexes to avoid row migration.

Run hitratio script to get the ratio.

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