Oracle: Get approximate size of storage used by indexes by a user

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SELECT   SEGMENT_NAME,   ROUND(BYTES / 1048576)       AS MEGABYTES,   ROUND(BYTES / 1073741824, 1) AS GIGABYTES FROM DBA_SEGMENTS WHERE SEGMENT_TYPE = 'INDEX' AND OWNER = '_YOURUSER_COMES_HERE_' ORDER BY 2 DESC, 1 ASC;