Oracle 11g分区表上的并发统计信息收集

Question

I am developing a DWH on Oracle 11g. We have some big tables (250+ million rows), partitioned by value. Each partition is a assigned to a different feeding source, and every partition is independent from others, so they can be loaded and processed concurrently.

Data distribution is very uneven, we have partition with millions rows, and partitions with not more than a hundred rows, but I didn't choose the partitioning scheme, and by the way I can't change it.

Considered the data volume, we must assure that every partition has always up-to-date statistics, because if the subsequent elaborations don't have an optimal access to the data, they will last forever.

So for each concurrent ETL thread, we

1. Truncate the partition
2. Load data from staging area with

SELECT /*+ APPEND */ INTO big_table PARTITION(part1) FROM temp_table WHERE partition_colum = PART1

(this way we have direct path and we don't lock the whole table)

3. We gather statistics for the modified partition.

In the first stage of the project, we used the APPROX_GLOBAL_AND_PARTITION strategy and worked like a charm

 dbms_stats.gather_table_stats(ownname=>myschema,
                              tabname=>big_table,
                              partname=>part1,
                              estimate_percent=>1,
                              granularity=>'APPROX_GLOBAL_AND_PARTITION',
                              CASCADE=>dbms_stats.auto_cascade,
                              degree=>dbms_stats.auto_degree) 

But, we had the drawback that, when we loaded a small partition, the APPROX_GLOBAL part was dominant (still a lot faster than GLOBAL) , and for a small partition we had, eg, 10 seconds of loading, and 20 minutes of statistics.

So we have been suggested to switch to the INCREMENTAL STATS feature of 11g, which means that you don't specify the partition you have modified, you leave all parameters in auto, and Oracle does it's magic, automatically understanding which partition(s) have been touched. And it actually works, we have really speeded up the small partition. After turning on the feature, the call became

 dbms_stats.gather_table_stats(ownname=>myschema,
                              tabname=>big_table,
                              estimate_percent=>dbms_stats.auto_sample_size,
                              granularity=>'AUTO',
                              CASCADE=>dbms_stats.auto_cascade,
                              degree=>dbms_stats.auto_degree) 

notice, that you don't pass the partition anymore, and you don't specify a sample percent.

But, we're having a drawback, maybe even worse that the previous one, and this is correlated with the high level of parallelism we have.

Let's say we have 2 big partition that starts at the same time, they will finish the load phase almost at the same time too.

1. The first thread ends the insert statement, commits, and launches the stats gathering. The stats procedure notices there are 2 partition modified (this is correct, one is full and the second is truncated, with a transaction in progress), updates correctly the stats for both the partitions.

2. Eventually the second partition ends, gather the stats, it see all partition already updated, and does nothing (this is NOT correct, because the second thread committed the data in the meanwhile).

The result is:

PARTITION NAME | LAST ANALYZED        | NUM ROWS | BLOCKS | SAMPLE SIZE
-----------------------------------------------------------------------
PART1          | 04-MAR-2015 15:40:42 | 805731   | 20314  | 805731
PART2          | 04-MAR-2015 15:41:48 | 0        | 16234  | (null)

and the consequence is that I occasionally incur in not optimal plans (which mean killing the session, refresh manually the stats, manually launch the precess again).

I tried even putting an exclusive lock on the gathering, so no more than one thread can gather stats on the same table at once, but nothing changed.

IMHO this is an odd behaviour, because the stats procedure, the second time it is invoked, should check for the last commit on the second partition, and should see it's newer than the last stats gathering time. But seems it's not happening.

Am I doing something wrong? Is it an Oracle bug? How can I guarantee that all stats are always up-to-date with incremental stats feature turned on, and an high level of concurrency?

Answer 1

I managed to reach a decent compromise with this function.

PROCEDURE gather_tb_partiz(
    p_tblname IN VARCHAR2,
    p_partname IN VARCHAR2)
IS
  v_stale all_tab_statistics.stale_stats%TYPE;
BEGIN
  BEGIN
    SELECT stale_stats
    INTO v_stale
    FROM user_tab_statistics
    WHERE table_name = p_tblname
    AND object_type = 'TABLE';
  EXCEPTION
  WHEN NO_DATA_FOUND THEN
    v_stale := 'YES';
  END;
  IF v_stale = 'YES' THEN
    dbms_stats.gather_table_stats(ownname=>myschema, 
                                  tabname=> p_tblname,
                                  partname=>p_partname,
                                  degree=>dbms_stats.auto_degree,
                                  granularity=>'APPROX_GLOBAL AND PARTITION') ;
  ELSE
    dbms_stats.gather_table_stats(ownname=>myschema,
                                 tabname=>p_tblname,
                                 partname=>p_partname,
                                 degree=>dbms_stats.auto_degree,
                                 granularity=>'PARTITION') ;
  END IF;
END gather_tb_partiz;

At the end of each ETL, if the number of added/deleted/modified rows is low enough not to mark the table as stale (10% by default, can be tuned with STALE_PERCENT parameter), I collect only partition statistics; otherwise i collect global and partition statistics.

This keeps ETL of small partition fast, because no global partition must be regathered, and big partition safe, because any subsequent query will have fresh statistics and will likely use an optimal plan.

Incremental stats is anyway enabled, so whenever the global has to be recalculated, it is pretty fast because aggregates partition level statistics and does not perform a full scan.

I am not sure if, with incremental enabled, "APPROX_GLOBAL AND PARTITION" and "GLOBAL AND PARTITION" do differ in something, because both incremental and approx do basically the same thing: aggregate stats and histograms without doing a full scan.

Answer 2

Have you tried to have incremental statistics on, but still explicitly name a partition to analyze?

 dbms_stats.gather_table_stats(ownname=>myschema,
                              tabname=>big_table,
                              partname=>part,
                              degree=>dbms_stats.auto_degree);

Answer 3

For your table, stale (yesterday's) global stats are not as harmful as completely invalid partition stats (0 rows). I can propose 2 a bit alternative approaches that we use:

• Have a separate GLOBAL stats gathering executed by your ETL tool right after all partitions are loaded. If it's taking too long, play with estimate_percent as dbms_stats.auto_degree will likely to be more than 1%
• Gather the global (as well as all other stale) stats in a separate database job run later during the day, after all data is loaded into DW.

The key point is that stale statistics which differ only slightly from fresh are almost just as good. If statistics show you 0 rows, they'll kill any query.

Answer 4

Considering what you are trying to achieve, you need to run stats on specific intervals of time for all Partitions and not at the end of the process that loads each partition. It could be challenging if this is a live table and has constant data loads happening round the clock, but since these are LARGE DW tables I really doubt that's the case. So the best bet would be to collect stats at the end of loading all partitions, this will ensure that the statistics is collected for partitions where data has change or statistics are missing and update the global statistics based on the partition level statistics and synopsis.

However to do so, you need to turn on incremental feature for the table (11gR1).

EXEC DBMS_STATS.SET_TABLE_PREFS('<Owner>','BIG_TABLE','INCREMENTAL','TRUE');

At the end of every load, gather table statistics using GATHER_TABLE_STATS command. You don't need to specify the partition name. Also, do not specify the granularity parameter.

EXEC DBMS_STATS.GATHER_TABLE_STATS('<Owner>','BIG_TABLE');

Answer 5

Kindly check if you have used DBMS_STATS to set table preference to gather incremental statistics. This oracle blog explains that statistics will be gathered after each row affected.

Incremental statistics maintenance needs to gather statistics on any partition that will change the global or table level statistics. For instance, the min or max value for a column could change after just one row is inserted or updated in the table

BEGIN 
DBMS_STATS.SET_TABLE_PREFS(myschema,'BIG_TABLE','INCREMENTAL','TRUE'); 
END;

Answer 6

I'm a bit rusty about it, so first of all a question: did you try serializing partition loading? If so, how long and how well does statistics run? Notice that since loading time is so much smaller than statistics gathering, i guess this could also act as a temporary workaround.

Append hint does affects redo size, meaning the transaction just traces something, thus statistics may not reckon new data: http://oracle-base.com/articles/misc/append-hint.php

Thinking out loud: since the direct path insert does append rows at the end of the partition and eventually updates metadata at the end, the already running thread gathering statistics could have read non-updated (stale) data. Thus it may not be a bug, and locking threads would accomplish nothing.

You may test this behaviour temporarily switching your table/partition to LOGGING, for instance, and see how it works (slower, of course, but it's a test). Can you do it?

EDIT: incremental stats should work anyway, even disabling a parallel statistics gathering, since it reiles on the incremental values no matter how they were collected: https://blogs.oracle.com/optimizer/entry/incremental_statistics_maintenance_what_statistics