A skinny table (also called a “Narrow Table” or a “Strip Table”) is a table that has very few columns and/or very narrow columns. This means the table has an small average row length. You can make a skinny table by paring away all columns that are unnecessary for querying from an existing table. This allows the table to have a narrower row size. Because the table has smaller row size, it allows more rows to fit within a single block.
If a table has more rows in a block, that means the table (ideally) occupies fewer blocks.
If the table is made up of fewer blocks, then less storage space is consumed and full table scans become cheaper.
An example of something similar to a “skinny table” is when Oracle performs a FAST FULL INDEX SCAN. In a FAST FULL INDEX SCAN, Oracle uses multi-block reads to read an index in lieu of reading the table. Oracle essentially treats the index as a skinnier version of the table.
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