Disk / Data read increase after putting on an index

I have a small query that runs pretty fast. And somehow I thought adding an index to an unindexed collumn would make it faster but turned out it didn't. In fact, it does increase my disk reads and execution time. What I'd like to ask is can someone explain me a detailed info about how the index works and why it could decrease performance rather than increase it.

Thanks in advance!

PS : My RDBMS : Oracle


Entirely possible on a small table. If the table is truly small it could be that the table can be read entirely into memory with a single read, and a full table scan can be performed entirely in memory. Adding an index here would require reading at least a single index page, followed by reading the data page, for a doubling of the I/O's. This is an unusual case but not unheard of.

However, this is just guesswork on my part. To truly find out what's going on grab the execution plan for your query with the index on, drop the index, and grab the execution plan without the index. Compare the plans, and decide if you want to re-add the index.

Share and enjoy.

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