DUAL was originally a #table and the #database engine would perform #disk IO on the table when selecting from #DUAL. This disk IO was usually logical IO (not involving physical disk access) as the disk blocks were usually already cached in memory. This resulted in a large amount of logical IO against the DUAL table.
The DUAL table is a special one-row table present by default in all Oracle database installations. It is suitable for use in selecting a #pseudocolumn such as #SYSDATE or #USER
The table has a single VARCHAR2(1) column called #DUMMY that has a value of “X”
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