Oracle Scratchpad

August 17, 2015

Index Usage

Filed under: extended stats,Indexing,Oracle,Tuning — Jonathan Lewis @ 4:25 pm BST Aug 17,2015

The question of how to identify indexes that could be dropped re-appeared (yet again) on the OTN database forum last week. It’s not really surprising that it recurs so regularly – the problem isn’t an easy one to solve but new (and even less new) users keep hoping that there’s a quick and easy solution.

There are, however, strategies and pointers that can help you to optimise the trade-off between effort, risk, and reward. Broadly the idea is to spend a small amount of effort finding a relatively small number of “expensive” indexes that might be safe to drop, so that when you do the detailed analysis you have a good chance that the time spent will be rewarded by a positive result.

Before we get to some results posted on OTN, it’s worth thinking about the global impact and what we’re trying to achieve, and the threats that go with our attempt to achieve it.

The key detail, of course, is that index maintenance is an expensive process. We could insert 1,000 rows into a table at a cost of writing about 25 table blocks plus a few undo blocks plus something like half a megabyte of redo (assuming, for the purposes of illustration that each row is about 200 bytes on insert). Add one index to the table and we might have to locate and modify 1,000 separate index leaf blocks. The increment on the redo might be about quarter of a megabyte and we may have to access 1,000 different undo blocks for read consistency reasons, but the simple fact that we may need 1,000 buffers to be able to maintain that index is likely to be a significant extra cost on the insert. Make that 10 indexes, or 70 (as one unhappy DBA once told me) and the probability of being able to do high-speed inserts becomes rather low.

Of course we hope that our indexes will allow our queries to operate efficiently with great precision, but inevitably we get to a point where the benefit of precision is outweighed by the cost of maintenance. Our target, then, is to design the set of indexes that makes it possible for the optimizer to find good paths for all the important queries and “good enough” paths for the rest. By the time the system is live, though, it’s too late for “proper design”, and the only option is for damage limitation, a bit of guesswork, and some live testing with fingers crossed (thank goodness for invisible indexes).

The starting point is usually an attempt to identify “the indexes we are not using”, which is typically translated into “the indexes that do not appear in execution plans” – but that’s not actually a good target, for various reasons:

  • Problem 1: If we are using an index it’s possible that we shouldn’t be and that there’s an alternative index available that ought to be more efficient. A corollary to this is that if you do identify and drop such an index you may find that the optimizer doesn’t use the alternative index you were expecting it to use until you take some action to help the optimizer recognise that the alternative is a good choice.
  • Problem 2: if we aren’t using a particular index then perhaps we should be using it and would use it if we dropped one of the other indexes on the table. (And there’s always the possibility that we didn’t happen to use it during the interval we were checking but do use it at some other times)
  • Problem 3: the optimizer is capable of using information about the number of distinct keys in a multi-column index to select an executon plan even though it may not use that index in the plan it finally chooses. We may be able to work around this problem in current versions of Oracle by creating a column group (extended statistics) that matches the definition of each index we drop – but there’s a limit of 20 column groups per table (and we may have to find the “opposite end” of each join where we use the index stats and create a matching column group there).
  • Problem 4: There are some indexes we might not be using but which must exist to avoid the “foreign key locking” problem. It should be easy enough to check, before dropping an index, whether it has to exist to match a foreign key; and even then it may be possible to show that nothing in the application would cause the locking problem to appear – and as a safety measure you could disable locks on the (child) table to ensure that the application doesn’t grind to a halt because of foreign key locking problems.

Provided you remember that problems like these exist, and think carefully about the indexes that your strategy suggests, there are various ways you could approach the problem of identifying indexes that don’t get into execution plans.

v$object_usage

The ink had barely dried on the manual pages for this view before several people (including me) had written notes explaining why this view wasn’t particularly helpful. (I think I even said something about this in Practical Oracle 8i). I won’t repeat the discussion here but it revolves around the fact that an index is flagged as “used” even if it has only been used once in a single execution of a single statement – so you don’t get any idea of the real importance of the index.

v$sql_plan et. al.

If you review the set of in-memory execution plans (and the AWR or Statspack equivalents) you can identify indexes which definitely have been used – but (a) it’s expensive to scan v$sql_plan frequently and (b) the AWR/Statspack repositories only capture a subset of the more expensive plans, so it’s easy to miss indexes which have been used and are relatively important but aren’t in the repository and don’t happen to be in memory at the moments you look.

Review the definitions

If you examine the index definitions you may spot indexes where look very similar. If one index starts with the same columns, in the same order, as another index, there is a good chance that you could reduce two indexes to one – especially if the whole of one of the indexes is the “leading edge” of the other – for example:

  • (dp_datetime_date)
  • (dp_datetime_date, dp_compid)

Even if the leading edges match and the trailing edges differ we might be able to collapse two indexes into one – depending on how selective the leading columns are and how the indexes are used – for example:

  • (dp_compid, ddzt, cirmhcx, ct_nxr_mhcx, dp_datetime_date)
  • (dp_compid, ddzt, cirmhcx, ct_nxr_mhcx, pnr_cfrqsj_date)

which could perhaps be replaced by one of :

  • (dp_compid, ddzt, cirmhcx, ct_nxr_mhcx, dp_datetime_date, pnr_cfrqsj_date)

or

  • (dp_compid, ddzt, cirmhcx, ct_nxr_mhcx, pnr_cfrqsj_date, dp_datetime_date)

Guessing about the use of a typical date column, though, it’s possible that in this example the current trailing date columns are used with a range-based predicate, so it’s possible that this strategy won’t be effective for this pair of indexes.

Even if the order of later columns in the index doesn’t match you may still find that a pair of indexes could be reduced to a single index – for example the pair:

  • (dp_datetime_date, dp_compid)
  • (dp_datetime_date, ddzdt, dp_compid, ct_nxrdh, ct_smsmobilno)

which could perhaps be replaced by just:

  • (dp_datetime_date, dp_compid, ddzdt, ct_nxrdh, ct_smsmobilno)

As a safety measure, of course, you would probably create a new index, then make the subject indexes invisible, and wait for at least a week to see whether any performance problems appear (remembering that one automatic performance threat would be the increase in workload as yet another index – temporarily – has to be maintained).

The difficulty of eliminating indexes by examination is that it takes a lot of effort to investigate all the possibilities, so you really need some way of choosing a relatively small subset of indexes that might be worth the effort. This brings me to the principle topic of this posting – using segment statistics to help you pick which indexes might be worth the effort.

v$segstat / v$segment_statistics

Oracle records a number of workload statistics for each object in memory. The view v$segstat is an efficient version of these statistics, and v$segment_statistics is a friendlier version that joins v$segstat to tables user$, obj$ and ts$, with a filter against ind$ to turn meaningless numbers into names.

SQL> desc V$segstat
 Name                    Null?    Type
 ----------------------- -------- ----------------
 TS#                              NUMBER
 OBJ#                             NUMBER
 DATAOBJ#                         NUMBER
 STATISTIC_NAME                   VARCHAR2(64)
 STATISTIC#                       NUMBER
 VALUE                            NUMBER

SQL> desc V$segment_statistics
 Name                    Null?    Type
 ----------------------- -------- ----------------
 OWNER                            VARCHAR2(30)
 OBJECT_NAME                      VARCHAR2(30)
 SUBOBJECT_NAME                   VARCHAR2(30)
 TABLESPACE_NAME                  VARCHAR2(30)
 TS#                              NUMBER
 OBJ#                             NUMBER
 DATAOBJ#                         NUMBER
 OBJECT_TYPE                      VARCHAR2(18)
 STATISTIC_NAME                   VARCHAR2(64)
 STATISTIC#                       NUMBER
 VALUE                            NUMBER

For each segment Oracle records the following statistics (according to v$segstat_name – but there are a couple more hidden statistics reported in the underlying x$ksolsstat object):

NAME                             SAMPLED
-------------------------------- -------
logical reads                    YES
buffer busy waits                NO
gc buffer busy                   NO
db block changes                 YES
physical reads                   NO
physical writes                  NO
physical read requests           NO
physical write requests          NO
physical reads direct            NO
physical writes direct           NO
optimized physical reads         NO
optimized physical writes        NO
gc cr blocks received            NO
gc current blocks received       NO
ITL waits                        NO
row lock waits                   NO
space used                       NO
space allocated                  NO
segment scans                    NO

Both Statspack (at level 7) and the AWR report have several “Top N” sections for segment statistics. If we examine these stats for all the indexes on a given table we can get some clues about which indexes are likely to be worth further investigation to see if they could be dropped.

One very simple measure is the number of “physical reads” (which, for indexes, will generally be very similar to “physical read requests”). Since a (real) physical read is generally going to take a significant amount of time, segments with very large numbers of physical reads could be contributing a lot of of time to the total database time – so it’s worth knowing why it’s responsible for so many physical reads and worth cross-checking with v$sql_plan (and its historic equivalents) which statements seem to be using or modifying this index.

Even if it turns out that the index is absolutely necessary, you might still be able to spot opportunities to improve efficiency. If it is subject to a significant number of physical reads it may be that the index is just very large – could you make it smaller by rebuilding it with compression on some of the leading columns, is it an index which (for some reason you can identify) tends to degenerate over time and waste a lot of space and should you rebuild it occasionally. It might be possible (depending on the predicates used) to re-arrange the column order in such a way that the activity is focused onto a particular section of the index rather than being spread across the entire index – or you could even find that by careful choice of global partitioning (which is legal on even a non-partitioned table) you might be able to isolate the activity to a small section of the index.

A more interesting measure, though, comes from comparing the “logical reads” with the number of “db block changes”; and that’s the point of this posting – except that I’ve spent so much time on it already that I’m going to have to write part 2 some time next week.

 

5 Comments »

  1. Thanks Jonathan for these nice insights.
    Foued

    Comment by fouedgray — August 17, 2015 @ 4:48 pm BST Aug 17,2015 | Reply

  2. Fantastic post as always from JL. I can’t wait for the sequel posts. Thanks.
    My guess is that JL will be correlating the number of index blocks that are brought to the Buffer Cache and being logically read (as an access path for the data block) with the amount of blocks data blocks that need to be changed. But, that is just a guess of mine.

    Comment by Daniel Da Meda — August 19, 2015 @ 3:11 pm BST Aug 19,2015 | Reply

  3. […] beeen a little slow in the follow-up to my previous posting on possibly redundant indexes. Before going into the slightly more complex stuff, there’s another peripheral point (but a […]

    Pingback by Index Usage – 2 | Oracle Scratchpad — August 29, 2015 @ 11:34 am BST Aug 29,2015 | Reply

  4. […] my last note on index usage I introduced the idea of looking at v$segstat (or v$segment_statistics) and comparing the […]

    Pingback by Index Usage – 3 | Oracle Scratchpad — September 1, 2015 @ 5:52 pm BST Sep 1,2015 | Reply


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