Oracle Storage Index

If you are on Exadata or taking advantage of Database In-Memory it’s possible your queries will benefit for the automatically created and maintained Storage Indexes.

But what exactly are Storage Indexes and why don’t I always see a benefit from them?

Let me start by describing what Storage Indexes are in relation to Database In-Memory but remember they behavior in exactly the same way on the Exadata storage cell.

A Storage Index keeps track of minimum and maximum values for each column in an In-Memory Compression Unit (IMCU) or 1MB chunk on the Exadata storage cells. When a query specifies a WHERE clause predicate, the In-Memory Storage Index on the referenced column(s) is examined to determine if any entries with the specified value exist.

If you are on Exadata or taking advantage of Database In-Memory it’s possible your queries will benefit for the automatically created and maintained Storage Indexes.

What are Storage Indexes?

Let me start by describing what Storage Indexes are in relation to Database In-Memory but remember they behavior in exactly the same way on the Exadata storage cell.

A Storage Index keeps track of minimum and maximum values for each column in an In-Memory Compression Unit (IMCU) or 1MB chunk on the Exadata storage cells. When a query specifies a WHERE clause predicate, the In-Memory Storage Index on the referenced column(s) is examined to determine if any entries with the specified value exist.

Continue reading “Oracle Storage Index”

Histogram sample size and Online Statistics Gathering

I got a great follow up question to my earlier blog on Online Statistics Gathering; that I thought might be of interest to other and worthy of a short post of its own.

The question related to sample size used to gather histograms on a table that originally had its statistics gathered via a direct path load operation.

Let’s look at any example:

I got a great follow up question to my earlier blog on Online Statistics Gathering; that I thought might be of interest to other and worthy of a short post of its own.

The question related to sample size used to gather histograms on a table that originally had its statistics gathered via a direct path load operation.

Let’s look at any example:

Continue reading “Histogram sample size and Online Statistics Gathering”