Why SYS. DBMS_PARALLEL_EXECUTE. CREATE_CHUNKS_BY_ROWID creates sometimes higher than expected pieces?

I am running

Oracle Database 11 g Enterprise Edition Release 11.2.0.3.0 - 64 bit Production

PL/SQL Release 11.2.0.3.0 - Production

"CORE 11.2.0.3.0 Production."

AMT for Linux: Version 11.2.0.3.0 - Production

NLSRTL Version 11.2.0.3.0 - Production

I build and maintain a data warehouse and often use DBMS_PARALLEL_EXECUTE for the loading of large amounts of data. I noticed that sometimes, especially when the size of my lot is large compared to the total number of rows in the segmented table, CREATE_CHUNKS_BY_ROWID creates a lot more than pieces that I would have expected, which most contain no data. For example, in a recent essay, I had 33K lines in the segmented table, and I specify a batch size of 10K. I expect about 4 pieces. I had 1002. All works fine, I'm just curious about these many (occasional) empty pieces. I always run a GATHER_STATS on the segmented table before DBMS_PARALLEL_EXECUTE.

I stumbled across this myself.

Simplified: the ROWID is discontinuous.

Why?:  Lines (re) move, space is created. (especially when you add LOB data<4000 bytes="" so="" that="" it="" is="" stored="">

If you do a join / select on the pieces, you will probably see most of them have count (*) 0

(something like that...)

Select C.chunk_id, count (*)

of user_parallel_execute_chunks C

une_table T

where T.rowid between C.start_rowid and C.end_rowid

C.chunk_id group;

I would only use CHUNK_BY_ROWID on the tables where I can assume that the data move and/or produce gaps during normal activity.

(for example warehouse tables)

TAKE 2:

I also ran into this problem where the KP has been very rare (and would produce the same kind of problem).

This would produce similar results when using CHUNK_BY_NUMBER_COLUMN().

To overcome that, I used an intermediate table and stored in bulk on the table.

(not for you to edit the SQL code used for PE)

for example

create the table intermediate_table

as

Select rownum, T_ID, T.some_pk

of une_table T.

MK

Tags: Database

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