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Applyinga blockcentricapproachtotuning
- 1. Applying a Blockcentric Approach to Oracle Tuning Daniel W. Fink www.optimaldba.com
- 12. High Water Mark H D D D D HWM
- 29. High Water Mark H D D D D HWM
Editor's Notes
- This is a very high level presentation. Intended to offer new ideas, a new approach. This
- Use the ideas here to begin to examine your systems. Each db system will have nuances that need to be addressed.
- A request for a single byte column in a single row will result in the entire block being accessed.
- A request for a single byte column in a single row will result in the entire block being accessed.
- Try this. Insert 1 million rows into an empty table, but rollback. Then do a select * from the table. Then truncate the table and rerun the query. Truncate resets the HWM.
- Statement 1 - Cost = 5100 + (100 * 10000) = 1,005,100 Statement 2 - Cost = 400 + (200 * 10000) = 2,000,400
- 65 blocks below HWM 200 rows per block 13,000 rows Query selects 100 rows or .77% of the rows Query reads 53 blocks or 82% of the blocks
- 65 blocks below HWM 200 rows per block 13,000 rows Query selects 100 rows or .77% of the rows Query reads 7 blocks or 11% of the blocks
- 65 blocks below HWM, but only 6 contain data 200 rows per block 1200 rows Query selects 1200 rows or 100% of the rows Query reads 6 blocks or 10% of the blocks
- How do you determine the # of blocks containing data? Not straightforward, if rows are chained or have migrated. Using dbms_rowid only returns the ‘head’ piece for a continued row.