Getting same index size despite different table size

Question

Hello,
this question arose from a different thread, but touches a different problem, which is why I have decided to post it as a separate thread.
I have several tables of 3D points.
The points roughly describe the same area but in different densities, which means the tables are of different sizes. The smallest contains around 3million entries and the largest around 37 million entries.
I applied an index with
CREATE INDEX <index name>
ON <table name>(<column name>)
INDEXTYPE is MDSYS.SPATIAL_INDEX
PARAMETERS('sdo_indx_dims=3');
My problem is that I am trying to see how much space the index occupies for each table.
I used the following syntax to get the answer to this:
SELECT usim.sdo_index_name segment_name, bytes/1024/1024 segment_size_mb
FROM user_segments us, user_sdo_index_metadata usim
WHERE usim.SDO_INDEX_NAME = <spatial index name>
AND us.segment_name = usim.SDO_INDEX_TABLE;
(thanks Reggie for supplying the sql)
Now, the curious thing is that in all cases, I get the answer
SEGMENT_NAME SEGMENT_SIZE_MB
LIDAR_POINTS109_IDX .0625
(obviously with a different sement name in each case).
I tried to see what an estimated index size would be with
SDO_TUNE.ESTIMATE_RTREE_INDEX_SIZE
And I get estimates ranging from 230MB in the case of 3million records up to 2.9 for the case of 37million records.
Does anyone have an idea why I am not getting a different actual index size for the different tables?
Any help is greatly appreciated!!!
Cheers,
F.

Answer 1

It looks like your indexes didn't actually create properly. Spatial indexes are a bit different to 'normal' indexes in this regard. A BTree index will either create or not. However, when creating a spatial index, something may fail, but the index structure will remain and it will appear to be valid according to the data dictionary.
Consider the following example in which the SRID has a problem:
SQL> CREATE TABLE INDEX_TEST (
  2  ID NUMBER PRIMARY KEY,
  3  GEOMETRY SDO_GEOMETRY);
Table created.
SQL>
SQL> INSERT INTO INDEX_TEST (ID, GEOMETRY) VALUES (1,
  2  SDO_GEOMETRY(2001, 99999, SDO_POINT_TYPE(569278.141, 836920.735, NULL), NULL, NULL)
  3
SQL> INSERT INTO user_sdo_geom_metadata VALUES ('INDEX_TEST','GEOMETRY',
  2     MDSYS.SDO_DIM_ARRAY(
  3     MDSYS.SDO_DIM_ELEMENT('X',0, 1000, 0.0005),
  4     MDSYS.SDO_DIM_ELEMENT('Y',0, 1000, 0.0005)
  5  ), 88888);
1 row created.
SQL>
SQL> CREATE INDEX INDEX_TEST_SPIND ON INDEX_TEST(GEOMETRY) INDEXTYPE IS MDSYS.SPATIAL_INDEX;
CREATE INDEX INDEX_TEST_SPIND ON INDEX_TEST(GEOMETRY) INDEXTYPE IS MDSYS.SPATIAL_INDEX
ERROR at line 1:
ORA-29855: error occurred in the execution of ODCIINDEXCREATE routine
ORA-13249: SRID 88888 does not exist in MDSYS.CS_SRS table
ORA-29400: data cartridge error
Error - OCI_NODATA
ORA-06512: at "MDSYS.SDO_INDEX_METHOD_10I", line 10
SQL> SELECT usim.sdo_index_name segment_name, bytes/1024/1024 segment_size_mb,
  2  usim.sdo_index_status
  2  FROM user_segments us, user_sdo_index_metadata usim
  3  WHERE usim.SDO_INDEX_NAME = 'INDEX_TEST_SPIND'
  4  AND us.segment_name = usim.SDO_INDEX_TABLE;
SEGMENT_NAME                     SEGMENT_SIZE_MB SDO_INDEX_STATUS
INDEX_TEST_SPIND                           .0625 VALID
1 row selected.
SQL>When you ran the CREATE INDEX statement did it say "Index created." afterwards or did you get an error?
Did you run the CREATE INDEX statement in SQL*Plus yourself or was it run by some software?
I suggest you drop the indexes and try creating them again. Watch out for any errors. Chances are its an SRID issue.