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Don Burleson Blog 







Tightly Packing Table Rows onto Data Blocks

Oracle Database Tips by Donald BurlesonJanuary 6, 2015

Question:  I have some read-only tables and I want to tightly pack the rows as dense as possible onto the data blocks in an optimal way.  There will be no updates, deletes or inserts.  How to you minimize space wastage and ensure fast retrieval via an index?

Answer:  There are several factors that govern free space within a table and a tablespace:

  • PCTFREE:  This is the percentage of space to leave free default (20%) for subsequent inserts or updates.  To pack the data tightly, set PCTFREE to a small value (e.g. 1).
  • NEXT extent size: A smaller NEXT extent size will minimize wastage.

To pack space tight, use these techniques:

STEP 1 - Load the table, adjusting PCTFREE and NEXT to minimize wastage.

STEP 2 - Reorganize the table (using CTAS with order by) into a fresh tablespace.  Re-sequencing the rows into the same order as the most-frequently-used index.  This reduces run-time I/O by fetching related rows in a single block read  The reorg table will have these features:

- Use insert append in the CTAS to always use a fresh empty block.

- Use table compression

- Make sure that there are no chained rows

- Re-set INITIAL to fit the whole table into one extent

- Set PCTFREE to a small value, to un-link the page only when it is very full.

To verify the dense row space packing you can run tablespace mapping scripts to see the free space on each block.

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