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Oracle DBA Interview Questions

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Oracle's SQL and PL/SQL engines are separate parts of the kernel which require context switching, like between unix processes. This is slow, and uses up resources. If we loop on an SQL statement, we are implicitely flipping between these two engines. We can minimize this by loading our data into an array, and using PL/SQL bulk binding operation to do it all in one go!

SQL*Loader with direct path option can load data ABOVE the high water mark of a table, and DIRECTLY into the datafiles, without going through the SQL engine at all. This avoids all the locking, latching, and so on, and doesn't impact the db (except possibly the I/O subsystem) at all.

Fewer indexes on a table mean faster inserts/updates. More indexes mean faster, more specific WHERE clauses possibly without index merges.

RAID 5 is striping with an extra disk for parity. If we lose a disk we can reconstruct from that parity disk.
RAID 10 is mirroring pairs of disks, and then striping across those sets.
RAID 5 was created when disks were expensive. Its purpose was to provide RAID on the cheap. If a disk fails, the IO subsystem will perform VERY slowly during the rebuild process. What's more your liklihood of failure increases dramatically during this period, with all the added weight of the rebuild. Even when it is operating normally RAID 5 is slow for everything but reading. Given that and knowing databases (especially Oracle's redo logs) continue to
RAID10 is just all around goodness. If you lose one disk in a set of 10 for example, you could lose any one of eight other disks and have no troubles. What's more rebuilding does not impact performance at all since you're simply making a mirror copy. Lastly RAID10 perform exceedingly well in all types of databases. 

Co-related sub query is one in which inner query is evaluated only once and from that result outer query is evaluated.
Nested query is one in which Inner query is evaluated for multiple times for getting one row of that outer query.
ex. Query used with IN() clause is Co-related query.
Query used with = operator is Nested query

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