Enabling Explain Plan at Session level

SQL>  set autotrace traceonly explain;
SQL>
SQL> select count(0) from tab;

Execution Plan
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Plan hash value: 1728992512

-----------------------------------------------------------------------------------
| Id  | Operation               | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |         |     1 |    44 |   106   (1)| 00:00:02 |
|   1 |  SORT AGGREGATE         |         |     1 |    44 |            |          |
|*  2 |   FILTER                |         |       |       |            |          |
|   3 |    NESTED LOOPS OUTER   |         |  1270 | 55880 |   106   (1)| 00:00:02 |
|*  4 |     HASH JOIN           |         |  1270 | 49530 |    31   (4)| 00:00:01 |
|   5 |      INDEX FULL SCAN    | I_USER2 |    93 |  2046 |     1   (0)| 00:00:01 |
|*  6 |      INDEX RANGE SCAN   | I_OBJ5  |  1270 | 21590 |    29   (0)| 00:00:01 |
|   7 |     TABLE ACCESS CLUSTER| TAB$    |     1 |     5 |     1   (0)| 00:00:01 |
|*  8 |      INDEX UNIQUE SCAN  | I_OBJ#  |     1 |       |     0   (0)| 00:00:01 |
|   9 |    NESTED LOOPS         |         |     1 |    29 |     2   (0)| 00:00:01 |
|* 10 |     INDEX SKIP SCAN     | I_USER2 |     1 |    20 |     1   (0)| 00:00:01 |
|* 11 |     INDEX RANGE SCAN    | I_OBJ4  |     1 |     9 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
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   2 - filter("O"."TYPE#"<>4 AND "O"."TYPE#"<>5 AND "O"."TYPE#"<>7 AND
              "O"."TYPE#"<>8 AND "O"."TYPE#"<>9 AND "O"."TYPE#"<>10 AND "O"."TYPE#"<>11
              AND "O"."TYPE#"<>12 AND "O"."TYPE#"<>13 AND "O"."TYPE#"<>14 AND
              "O"."TYPE#"<>22 AND "O"."TYPE#"<>87 AND "O"."TYPE#"<>88 OR
              BITAND("U"."SPARE1",16)=0 OR ("O"."TYPE#"=4 OR "O"."TYPE#"=5) AND
              ("U"."TYPE#"<>2 AND SYS_CONTEXT('userenv','current_edition_name')='ORA$BASE
              ' OR "U"."TYPE#"=2 AND "U"."SPARE2"=TO_NUMBER(SYS_CONTEXT('userenv','curren
              t_edition_id')) OR  EXISTS (SELECT 0 FROM SYS."USER$" "U2",SYS."OBJ$" "O2"
              WHERE "O2"."OWNER#"="U2"."USER#" AND "O2"."TYPE#"=88 AND
              "O2"."DATAOBJ#"=:B1 AND "U2"."TYPE#"=2 AND
              "U2"."SPARE2"=TO_NUMBER(SYS_CONTEXT('userenv','current_edition_id')))))
   4 - access("O"."OWNER#"="U"."USER#")
   6 - access("O"."SPARE3"=USERENV('SCHEMAID') AND "O"."TYPE#">=2 AND
              "O"."LINKNAME" IS NULL AND "O"."TYPE#"<=5)
       filter("O"."TYPE#"<=5 AND "O"."TYPE#">=2 AND "O"."LINKNAME" IS NULL)
   8 - access("O"."OBJ#"="T"."OBJ#"(+))
  10 - access("U2"."TYPE#"=2 AND "U2"."SPARE2"=TO_NUMBER(SYS_CONTEXT('usere
              nv','current_edition_id')))
       filter("U2"."TYPE#"=2 AND "U2"."SPARE2"=TO_NUMBER(SYS_CONTEXT('usere
              nv','current_edition_id')))
  11 - access("O2"."DATAOBJ#"=:B1 AND "O2"."TYPE#"=88 AND
              "O2"."OWNER#"="U2"."USER#")

SQL>