Re: Slow create table statement

Have I missed some posts on this thread ? It looks to me as if your problem is simply a bad execution plan - possibly due to bad stats.

The "inc" distinct count is about 1,700 but your total row count is 14M, which means you're using an index to pick up about 8,000 randomly distributed rows from a big table/ for each distinct value in the nested loop driver. The optimizer thinks (for whatever reason) it will be about 1,600 rows fairly well clustered (about 8 rows per block accessed). At 14M rows that's about 1.75M random I/Os - and there's little doubt that a lot of them will collide and most of them will be real physical reads.

How many partitions in the table and what is the block size ? (It looks like it might be 16KB blocks - and that might have an effect on performance).

You're optimum plan might be a parallel hash join with a broadcast/none distribution to minimise messaging between parallel execution processes.

One thing that might make the processing slow is your comment that you've got 30 columns of varchar2(4000) - even though the average row size seems to be in the region of 200 or 400 bytes - so maybe you've also got compression in place. One feature of very long rows is that if the declared length of the rows is greater than the size of a block Oracle will probably fall back to single row processing on the insert. (I'd have to check a combination of things to be sure, including the effect of the /*+ append */ hint). So it's almost certainly a good idea to pre-declare the target table - ideally limiting the column sizes to get a row size less than the block size.

Regards
Jonathan Lewis

On Fri, Jul 10, 2020 at 6:12 PM Ram Raman <veeeraman_at_gmail.com> wrote:

> Thanks Andy. I am not sure about a covering index as the table has
> around 50 columns, with 30 of them VARCHAR2(4000)! Not sure if creating a
> covering index on such a table with 2B rows a good idea.
>
> I did some tests and as you pointed out the count(*) comes back fast, but
> when i do a SELECT of all columns instead of count from the big table the
> query slows down to about 3 hours like the create table. I guess it is time
> for new hardware.
>
>
> On Wed, Jul 8, 2020 at 2:09 AM Andy Sayer <andysayer_at_gmail.com> wrote:
>
>> Hi Ram,
>>
>> Covering index means that it covers every column that’s required by the
>> query (even the ones in your select). Judging by the cost in the plan, it’s
>> unlikely the index used contained more than one column. Covering indexes
>> remove the need for table access. You would need to weigh up the cost of
>> creating an index the size of the table and the reward (this query gets
>> faster).
>>
>> Thanks,
>> Andrew
>>
>> On Wed, 8 Jul 2020 at 03:25, Ram Raman <veeeraman_at_gmail.com> wrote:
>>
>>> Thank you Tim and Andy,
>>>
>>> " besides making a covering index" Andy, there is a covering index which
>>> makes the access of CLN_EVNT table by index, per the plan
>>>
>>> On Tue, Jul 7, 2020 at 5:39 PM Andy Sayer <andysayer_at_gmail.com> wrote:
>>>
>>>> Tim,
>>>>
>>>> "Load as select" means it's doing direct path load. The additional step
>>>> of "OPTIMIZER STATISTICS GATHERING" also only appears for direct path load.
>>>> We can also tell that the insertion into the table is done in parallel
>>>> as we are still using PCWP in the IN-OUT column, it only serializes about
>>>> the load.
>>>>
>>>> Thanks,
>>>> Andy
>>>>
>>>> On Tue, 7 Jul 2020 at 23:25, Tim Gorman <tim.evdbt_at_gmail.com> wrote:
>>>>
>>>>> Ram,
>>>>>
>>>>> Perhaps it is serializing on the INSERT phase of the CTAS, keeping in
>>>>> mind that the CREATE clause both creates the table and does the insert?
>>>>> Perhaps...
>>>>>
>>>>> ALTER SESSION ENABLE PARALLEL DDL;
>>>>> CREATE TABLE ... PARALLEL 8 SELECT /*+ full(ce) parallel(ce,8)
>>>>> */ CE.* FROM ... ;
>>>>>
>>>>> Unless you are required to perform CTAS in one statement, could you
>>>>> perhaps split it into two steps, the first to create the table and the
>>>>> second to populate it? So as an alternative...
>>>>>
>>>>> CREATE TABLE .... SELECT * FROM IX.S_CLN_EVNT WHERE 1 = 0;
>>>>> ALTER SESSION ENABLE PARALLEL DML;
>>>>> INSERT /*+ APPEND PARALLEL(x,8) */ INTO REQ_27333_CLN_EVENT
>>>>> SELECT /*+ full(ce) parallel(ce,8) */ CE.* FROM ... ;
>>>>>
>>>>> This way you can use the APPEND hint to force direct-path INSERTs
>>>>> which are far more efficient than conventional INSERTs; I'm not sure if
>>>>> LOAD AS SELECT (HYBRID TSM/HWMB)" is direct-path or conventional? At any
>>>>> rate, with an APPEND hint you can be certain. You'll also get to specify
>>>>> the PARALLEL hint on the INSERT statement as well as the SELECT statement,
>>>>> to make sure of PX pipelining. Also, not sure if your session is enabled
>>>>> for parallel DDL or parallel DML, but if you break it into two operations,
>>>>> you will only need parallel DML.
>>>>>
>>>>> Hope this helps?
>>>>>
>>>>> -Tim
>>>>>
>>>>>
>>>>> On 7/7/2020 2:48 PM, Ram Raman wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> We are trying to create a table by pulling information from a big
>>>>> table, the create statement is taking too long. However when I run select
>>>>> count(*) with the same query it comes back in seconds. While the CREATE
>>>>> statement is running I see waits on parallel read almost all the time,
>>>>> however, I do not see the same waits during select count(*).
>>>>>
>>>>> Tried a few variations with parallelism degree in the Create
>>>>> statement. Also tried inserting after truncating the table, used INSERT
>>>>> with APPEND hint, same slow results. v12c. FORCE_LOGGING is not on at DB
>>>>> level, inserted table in nologging mode. During the run, the IO on the data
>>>>> disks is about 50 to 70% util. 2 cores are consistently at 99-100% (most of
>>>>> them are %iowait at ~95%) with the other cores idle during this run the
>>>>> last time I checked - I only checked the individual core usage this last
>>>>> time it was run. I was checking average before. The wait events are the
>>>>> same in all cases.
>>>>>
>>>>> The attached link
>>>>> <https://drive.google.com/file/d/1N8re2eAWLl0FHYdGs0c9I93e5ruoSNN6/view?usp=sharing>
>>>>> has all the commands and the information. Any help is appreciated.
>>>>>
>>>>> Thanks,
>>>>> Ram.
>>>>>
>>>>> --
>>>>>
>>>>>
>>>>>
>>>>>
>>>
>>> --
>>>
>>>
>>>
>
> --
>
>
>

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