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Logical and physical IO with respect to the CBO
By default, the CBO weighs logical io as expensive as physical io. This behaviour can be changed by adjusting optimizer_index_caching and optimizer_index_cost_adj.
The default value of 100 means that an index access to a table is just as expensive as a full table scan.
On most systems, the default value of 100 should be lowered.
This value is a percentage that indicates how many blocks are found in the buffer cache when an index is read.
This value is only for index blocks, not for blocks in the table that the index points to.
On most systems, the default value of 0 is too low and should be altered.