Data … as usual

Yesterday, I read a blog post from Maria Colgan (https://sqlmaria.com/2017/02/21/oracle-storage-index/) who described why you will not always see the benefits of In memory Storage Indexes.

I won’t re-write Maria’s post but to sum up, Oracle doesn’t sort the data to build IMCUs, as a result min and max values of the storage indexes are not very selective. I’ve explained this in my SIMD related presentation (available here: https://www.slideshare.net/lolo115/ukoug15-simd-outside-and-inside-oracle-12c-12102, See. Slides 22 and 23).

To go further, we can find a bunch of views, oops a bunch of undocumented views, related to IM segments, Compression units and SMUs.  (See doc bug in MOS: Bug 19361690 : SEVERAL V$ VIEWS FOR INMEMORY ARE NOT DOCUMENTED, Bug doesn’t seem to be fixed in 12.2)

If you have a look at these views, they contains a lot of very interesting things, and specially the min and max values for every IMCUs and columns inside.

In the below example, I took the same kind of example that Maria described.

First, I created two tables loaded in the IM store. Those tables are based on SH.SALES table, the first one is not sorted, the second is ordered by AMOUNT_SOLD :

SQL> create table s inmemory no memcompress priority critical as select * from sh.sales;

SQL> create table s2 inmemory no memcompress priority critical as select * from sh.sales order by amount_sold;

SQL> @IM_seg
Enter value for owner: LAURENT
Enter value for segment_name:

OWNER       SEGMENT_NAME    PARTITION_NAM TABLESPACE_NAME INMEMORY_SIZE      BYTES BYTES_NOT_POPULATED POPULATE_ INMEMORY INMEMORY_COMPRESS
----------- --------------- ------------- --------------- ------------- ---------- ------------------- --------- -------- -----------------
LAURENT     S                             USERS                28966912   37748736                   0 COMPLETED CRITICAL NO MEMCOMPRESS
LAURENT     S2                            USERS                28966912   37748736                   0 COMPLETED CRITICAL NO MEMCOMPRESS
                                                          -------------
sum                                                            57933824

Then, I used a script of mine based on one of these undocumented stuff, the V$IM_COL_CU view that gives us min and max values for each column in the IMCUs.

SQL> @IM_IMCU_stats
Enter value for tab_owner: LAURENT
Enter value for tab_name: S
Enter value for column_name: AMOUNT_SOLD

IMCU_ADDR                  OBJD ONAME                COLUMN_NAME                    DATA_TYPE  DICTIONARY_ENTRIES MIN_VAL         MAX_VAL
-------------------- ---------- -------------------- ------------------------------ ---------- ------------------ --------------- ---------------
00000000610FFF70          94467 S                    AMOUNT_SOLD                    NUMBER                      0 7.14            1520.39
00000000613FFF70          94467 S                    AMOUNT_SOLD                    NUMBER                      0 7.22            1533.38
00000000616FFF70          94467 S                    AMOUNT_SOLD                    NUMBER                      0 6.83            1566.01
00000000619FFF70          94467 S                    AMOUNT_SOLD                    NUMBER                      0 6.4             1598.63
0000000061CFFF70          94467 S                    AMOUNT_SOLD                    NUMBER                      0 6.54            1738.43
0000000076000000          94467 S                    AMOUNT_SOLD                    NUMBER                      0 6.54            1738.43
0000000076300000          94467 S                    AMOUNT_SOLD                    NUMBER                      0 6.54            1782.72
0000000076600000          94467 S                    AMOUNT_SOLD                    NUMBER                      0 7.13            1782.72
0000000076900000          94467 S                    AMOUNT_SOLD                    NUMBER                      0 7.13            1753.2

9 rows selected.

-- -------------------------------------

SQL> @IM_IMCU_stats
Enter value for tab_owner: LAURENT
Enter value for tab_name: S2
Enter value for column_name: AMOUNT_SOLD

IMCU_ADDR                  OBJD ONAME                COLUMN_NAME                    DATA_TYPE  DICTIONARY_ENTRIES MIN_VAL         MAX_VAL
-------------------- ---------- -------------------- ------------------------------ ---------- ------------------ --------------- ---------------
0000000071FFFFE8          94469 S2                   AMOUNT_SOLD                    NUMBER                      0 481             1782.72
00000000721FFFE8          94469 S2                   AMOUNT_SOLD                    NUMBER                      0 69.08           481
00000000724FFFE8          94469 S2                   AMOUNT_SOLD                    NUMBER                      0 51.43           69.08
00000000727FFFE8          94469 S2                   AMOUNT_SOLD                    NUMBER                      0 46.31           51.43
0000000072AFFFE8          94469 S2                   AMOUNT_SOLD                    NUMBER                      0 32.35           46.31
0000000072DFFFE8          94469 S2                   AMOUNT_SOLD                    NUMBER                      0 24.24           32.35
00000000730FFFE8          94469 S2                   AMOUNT_SOLD                    NUMBER                      0 17.79           24.24
00000000733FFFE8          94469 S2                   AMOUNT_SOLD                    NUMBER                      0 10.79           17.79
00000000736FFFE8          94469 S2                   AMOUNT_SOLD                    NUMBER                      0 6.4             10.79

9 rows selected.

We can clearly see that S2 table has been ordered by AMOUNT_SOLD, and if I count the number of lines with a value of AMOUNT_SOLD equal to 20, S table (unordered) will read all IMCUs (9), a query againt S2 will prune 8 of the 9 IMCUs.

SQL> @IM_sesstat

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
CPU used by this session                                                  3
IM scan CUs no memcompress                                                0
IM scan CUs memcompress for dml                                           0
IM scan CUs memcompress for query low                                     0
IM scan CUs memcompress for query high                                    0
IM scan CUs memcompress for capacity low                                  0
IM scan CUs memcompress for capacity high                                 0
IM scan CUs columns accessed                                              0
IM scan CUs columns decompressed                                          0
IM scan CUs columns theoretical max                                       0
IM scan rows                                                              0
IM scan rows valid                                                        0
IM scan rows optimized                                                    0
IM scan rows projected                                                    0
IM scan CUs split pieces                                                  0
IM scan CUs pruned                                                        0

16 rows selected.

SQL> select /*+ INMEMORY */ count(*) from s where amount_sold=20;

  COUNT(*)
----------
       140

1 row selected.

SQL> @IM_sesstat

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
CPU used by this session                                                 20
IM scan CUs no memcompress                                                9
IM scan CUs memcompress for dml                                           0
IM scan CUs memcompress for query low                                     0
IM scan CUs memcompress for query high                                    0
IM scan CUs memcompress for capacity low                                  0
IM scan CUs memcompress for capacity high                                 0
IM scan CUs columns accessed                                              9
IM scan CUs columns decompressed                                          0
IM scan CUs columns theoretical max                                      63
IM scan rows                                                         918843
IM scan rows valid                                                   918843
IM scan rows optimized                                                    0
IM scan rows projected                                                  140
IM scan CUs split pieces                                                  9
IM scan CUs pruned                                                        0

16 rows selected.

-- ---------------------------------------------

SQL> @IM_sesstat

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
CPU used by this session                                                  1
IM scan CUs no memcompress                                                0
IM scan CUs memcompress for dml                                           0
IM scan CUs memcompress for query low                                     0
IM scan CUs memcompress for query high                                    0
IM scan CUs memcompress for capacity low                                  0
IM scan CUs memcompress for capacity high                                 0
IM scan CUs columns accessed                                              0
IM scan CUs columns decompressed                                          0
IM scan CUs columns theoretical max                                       0
IM scan rows                                                              0
IM scan rows valid                                                        0
IM scan rows optimized                                                    0
IM scan rows projected                                                    0
IM scan CUs split pieces                                                  0
IM scan CUs pruned                                                        0

16 rows selected.

SQL> select /*+ INMEMORY */ count(*) from s2 where amount_sold=20;

  COUNT(*)
----------
       140

SQL> @IM_sesstat

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
CPU used by this session                                                  2
IM scan CUs no memcompress                                                9
IM scan CUs memcompress for dml                                           0
IM scan CUs memcompress for query low                                     0
IM scan CUs memcompress for query high                                    0
IM scan CUs memcompress for capacity low                                  0
IM scan CUs memcompress for capacity high                                 0
IM scan CUs columns accessed                                              1
IM scan CUs columns decompressed                                          0
IM scan CUs columns theoretical max                                      63
IM scan rows                                                         918843
IM scan rows valid                                                   103923
IM scan rows optimized                                               814920
IM scan rows projected                                                  140
IM scan CUs split pieces                                                  9
IM scan CUs pruned                                                        8

16 rows selected.

All the scripts I used in the post, including the one used to see storage indexes, are available here:

• IM_seg.sql
• IM_IMCU_stats.sql
• IM_sesstat.sql

That’s all for today !! 🙂