Oracle 11GR2 - When will datafile autoextend?
Extents will spread evenly:

SQL&gt; select * from v$version where rownum = 1;

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Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

SQL&gt; create tablespace test datafile size 10M autoextend on next 10M maxsize 100M;

Tablespace created.

SQL&gt; create table t1 (id number) tablespace test;

Table created.

SQL&gt; alter table t1 allocate extent (size 50M);

Table altered.

SQL&gt; select * from dba_tablespace_usage_metrics where tablespace_name = 'TEST';

TABLESPACE_NAME                USED_SPACE TABLESPACE_SIZE USED_PERCENT
------------------------------ ---------- --------------- ------------
TEST                                 6536           12800      51.0625

SQL&gt; alter tablespace test add datafile size 10M autoextend on next 10M maxsize 100M;

Tablespace altered.

SQL&gt; select file_id, sum(blocks) from dba_extents where tablespace_name = 'TEST' group by file_id;

   FILE_ID SUM(BLOCKS)
---------- -----------
         7        6408

SQL&gt; alter table t1 allocate extent (size 30M);

Table altered.

SQL&gt; select file_id, sum(blocks) from dba_extents where tablespace_name = 'TEST' group by file_id;

   FILE_ID SUM(BLOCKS)
---------- -----------
         8        1536
         7        8712