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CHAPTER 10 DATABASE TABLES
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The hash cluster query took the same amount of CPU for all intents and purposes, even though it went to the buffer cache one-third as many times. This, too, could be anticipated. The act of performing a hash is very CPU-intensive. The act of performing an index lookup is I/O-intensive. It was a tradeoff. However, as we scale up users, we would expect the hash cluster query to scale better, as it has to get in line to access the buffer cache less frequently.
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This last point is the important one. When working with computers, it is all about resources and their utilization. If we are I/O bound and perform queries that do lots of keyed reads like I just did, a hash cluster may improve performance. If we are already CPU bound, a hash cluster may possibly decrease performance since it needs more CPU horsepower to hash. However, if the extra CPU we are burning is due to spinning on cache buffers chains latches, the hash cluster could significantly reduce the CPU needed. This is one of the major reasons why rules of thumb do not work on real-world systems: what works for you might not work for others in similar but different conditions. There is a special case of a hash cluster called a single table hash cluster. This is an optimized version of the general hash cluster we ve already looked at. It supports only one table in the cluster at a time (you have to DROP the existing table in a single table hash cluster before you can create another). Additionally, if there is a one-to-one mapping between hash keys and data rows, the access to the rows is somewhat faster as well. These hash clusters are designed for those occasions when you want to access a table by primary key and do not care to cluster other tables with it. If you need fast access to an employee record by EMPNO, a single table hash cluster might be called for. I did the preceding test on a single table hash cluster as well and found the performance to be even better than just a hash cluster. You could even go a step further with this example and take advantage of the fact that Oracle will allow you to write your own specialized hash function (instead of using the default one provided by Oracle). You are limited to using only the columns available in the table, and you may use only the Oracle built-in functions (e.g., no PL/SQL code) when writing these hash functions. By taking advantage of the fact that OBJECT_ID is a number between 1 and 75,000 in the preceding example, I made my hash function simply be the OBJECT_ID column itself. In this fashion, I am guaranteed to never have a hash collision. Putting it all together, I ll create a single table hash cluster with my own hash function via: ops$tkyte@ORA11GR2> create cluster hash_cluster 2 ( hash_key number(10) ) 3 hashkeys 75000 4 size 150 5 single table 6 hash is HASH_KEY 7 / Cluster created. I ve simply added the key words SINGLE TABLE to make it a single table hash cluster. My HASH IS function is just the HASH_KEY cluster key in this case. This is a SQL function, so I could have used trunc(mod(hash_key/324+278,555)/abs(hash_key+1)) if I wanted (not that this is a good hash function it just demonstrates that we can use a complex function there if we wish). I used a NUMBER(10) instead of just a number. Since the hash value must be an integer, it cannot have any fractional components. Then, I create the table in that cluster ops$tkyte@ORA11GR2> create table t_hashed 2 cluster hash_cluster(object_id) 3 as 4 select OWNER, OBJECT_NAME, SUBOBJECT_NAME, 5 cast( OBJECT_ID as number(10) ) object_id, 6 DATA_OBJECT_ID, OBJECT_TYPE, CREATED, 7 LAST_DDL_TIME, TIMESTAMP, STATUS, TEMPORARY, 8 GENERATED, SECONDARY
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