Some time ago I had two questions about fixed objects statistics for which I couldn’t quickly find the answers. Questions are:
- how to gather fixed object stats for a given X$
- should I gather fixed objects stats on both RAC nodes
- why I can’t gather stats on some X$-tables
At the time of asking these question I’ve read CBO development team blog post on the topic, but still I was unable to answer them distinctly. Well, it appears the questions are simple and could probably be deduced after careful reading (first two questions for sure). The core phrase is:
The DBMS_STATS.GATHER_FIXED_OBJECTS_STATS procedure gathers the same statistics as DBMS_STATS.GATHER_TABLE_STATS except for the number of blocks
That’s the key, the statistics are the same, except for the number of blocks. Which effectively means, you can gather the statistics on the specific X$-table in the same way as you do with a normal table, by using DBMS_STATS.GATHER_TABLE_STATS procedure. The only difference would be you will not populate the number of blocks (but you can do this manually). Now, since these statistics are essentially the same as statistics for normal tables, it makes no sense to run the GATHER_FIXED_OBJECTS_STATS procedure on all RAC nodes. After all, the database is still the same and it doesn’t care about the table statistics in relation to RAC node (at least for now in 11g).
I wish I was that smart back then :) I’ve found the answers recently via MOS notes updates:
In RAC, Should We Execute GATHER_FIXED_OBJECTS_STATS And Gather_dictionary_stats In Both Nodes? [ID 1479804.1] which gives the definitive answer “No” and
Why do some Fixed Tables not have Optimizer Statistics [ID 1355608.1] which explains that some fixed tables cannot have optimizer statistics by design decisions made by Oracle.
What I don’t understand after getting better idea of fixed stats is why Oracle made the choice to introduce a specific new procedure, without any parameters/configuration options if they could have better done it inside the standard procedure for statistics gathering? To me it looks very strange and I would prefer not to introduce one more complication to the already over-complicated statistics package.