How big is an Oracle XMLType when stored as BINARY XML

Oracle Binary XML format corresponds to "Compact Schema Aware XML Format" abbreviated as CSX. Encoded data stored as BLOB field. Details about binary XML format available from Oracle documentation (here and here).

Real size of data field depends on LOB storage parameters of XMLType column. E.g. if storage in row option enabled then small documents stored directly with other data and vsize() returns appropriate values.

In reality Oracle creates underlying BLOB column with system name, which can be found by querying user_tab_cols view:

select table_name, column_name, data_type from user_tab_cols where   table_name = 'T' and hidden_column = 'YES'  and  column_id = (      select column_id       from user_tab_cols       where table_name = 'T' and column_name = 'X'  ) 

This query returns system hidden column name which looks like SYS_NC00002$.

After that it's possible to get size of fields with regular dbms_lob.getlength() call against hidden column:

select dbms_lob.getlength(SYS_NC00002$) from t

Actual storage consumption is stored in a view called user_segments. To find the correlating LOB to the column you will have to join user_segments with user_lobs:

CREATE TABLE clob_table (x XMLTYPE) XMLTYPE x store as CLOB;CREATE TABLE binaryxml_table (x XMLTYPE) XMLTYPE x STORE AS BINARY XML;INSERT INTO clob_table (x) SELECT  XMLELEMENT("DatabaseObjects",    XMLAGG(      XMLELEMENT("Object", XMLATTRIBUTES(owner, object_type as type, created, status), object_name)    )  ) as xFROM all_objects;INSERT INTO binaryxml_table (x) select  XMLELEMENT("DatabaseObjects",    XMLAGG(      XMLELEMENT("Object", XMLATTRIBUTES(owner, object_type as type, created, status), object_name)    )  ) as xFROM all_objects;SELECT lobs.table_name,  (SELECT column_name     FROM user_tab_cols       WHERE table_name = lobs.table_name AND data_type = 'XMLTYPE'  AND column_id =         (SELECT column_id            FROM user_tab_cols              WHERE table_name = lobs.table_name AND column_name = lobs.column_name          )    ) column_name,  seg.segment_name, seg.bytes    FROM user_lobs lobs, user_segments seg      WHERE lobs.segment_name = seg.segment_name;TABLE_NAME      COLUMN_NAME SEGMENT_NAME                 BYTES--------------- ----------- ------------------------- --------BINARYXML_TABLE X           SYS_LOB0000094730C00002$$  7536640 CLOB_TABLE      X           SYS_LOB0000094727C00002$$ 19922944 

[rep issue, not allowed to post comments]you wanted to say "between questions" as i understood. the only similarity is storage space issue, thought it might be helpful for "guess" estimation. you didn't mention what type of data you are going to store as bXML.

unpacks the binary XML into text

If pure XML then it depends on what compressor you are going to use. Usually lzma|gzip is used for binary compression. Maybe I am writing about too obvious things, but that's all I know