Here is a summary of the findings while evaluating Incremental Partition Statistics that have been introduced in Oracle 11g.
The most important point to understand is that Incremental Partition Statistics are not “cost-free”, so anyone who is telling you that you can gather statistics on the lowest level (partition or sub-partition in case of composite partitioning) without any noticeable overhead in comparison to non-incremental statistics (on the lowest level) is not telling you the truth. Although this might be obvious I’ve already personally heard someone making such claims so it’s probably worth to mention.
In principle you need to test on your individual system whether the overhead that is added to each statistics update on the lowest level outweighs the overhead of actually gathering statistics on higher levels, of course in particular on global level. This might also depend on your strategy how and how often you used to gather statistics so far. The overhead introduced by Incremental Partition Statistics can be significant, in terms of both runtime and data volume. You can expect the SYSAUX tablespace to grow by several GBs (for larger databases in the TB range easily in the tenth of GBs) depending on the number of partitions, number of columns and distinct values per column.
To give you an idea here are some example figures from the evaluation:
Table 1: 4 million total rows, 1 GB total size, 6 range partitions, 155 columns Table 2: 200 million total rows, 53 GB total size, 629 range-list subpartitions, 104 columns
For Table 1 Incremental stats maintained 700,000 rows in SYS.WRI$_OPTSTAT_SYNOPSIS$. For Table 2 it was 3,400,000 rows. In total for these two tables approx. 4.1 million rows and 170 MB had to be maintained in the SYS.WRI$_OPTSTAT_SYNOPSIS$ tables.
When I first saw this significant data volume generated for the synopsis meta data I was pretty sure that processing that amount of data will clearly cause some significant overhead, too.
And that is exactly what happens – for example a recursive DELETE statement on the SYS.WRI$_OPTSTAT_SYNOPSIS$ table took about 10 secs out of 16 secs total runtime of statistics gathering for a rather small partition of above partitioned table.