Oracle分区性能

Question

I have a large oracle table with more than 600 million records and we just repartitioned them so that we can purge some of the old data with out increasing the logs size.

Problem is there there are some queries that does full index scan and that are run very often like 300 times per sec. Before partition query used to take about .15 sec but after partition its taking .50 sec to 1.25 sec. Does anyone know partitioning oracle table degrades the performance of the query? If yes, could you give the reason? There seems to be some articles but not clear enough for me to understand.

Answer 1

If the index is local and the query is not based on the partitioning key (meaning: partition pruning is not possible) but highly selective the effort will increase in proportion to the number of partitions you create. If you have 30 partitions then 30 indexes have to be searched for your values. The fact that each index is smaller is not offset by the larger number of indexes. (You might want to look at how btree indexes work to understand why this is not the case).

To cut a long story short: If you use a global index you should be able to avoid this problem.

Answer 2

When having partition table and if you are having lot of select query on this table, always include in WHERE clause Paritioncoloumn =value.

example if the partition is based on column of date type (PERSISTED_DATE)

Query

SELECT * FROM TABLE_NAME WHERE COLOUMN1='VALUE' and trunc(PERSISTED_DATE)=trunc(sysdate);

Important points to note.

1. Avoid using global index if it is high transnational table else you have to build the global index after deletion of partition.
2. For better performance keep the partition count less, you can automate creating a new partition and deletion of older partition on daily basis.