Wait Event – latch: redo copy

Redo copy latches are acquired by sessions in order to write changes into the redo log buffer and by the LGWR process to ensure that there are no current writers before it copies data from the buffer to the redo log.

Wait Event – index block split

While trying to find an index key in an index block, Oracle noticed that the index block was being split. Oracle will wait for the split to finish and try to find the key again. Wait Time: The session will yield the CPU, so there is no actual waiting time   Parameter Description rootdba The root…

Index Skip Scans

An index skip scan occurs when the initial column of a composite index is “skipped” or not specified in the query. Basically skip scanning lets a composite index be split logically into smaller sub-indexes, making every sub-index a viable candidate to be used by optimizer to come up with an efficient plan. The number of logical…

Index Full Scans

As the name suggests, Index Full Scans are Index Scans where the entire index is scanned in order. Following are the situations where optimizer chooses Index Full Scans:- A predicate references a column in the index. This column need not be the leading column. No predicate is specified and all columns in the table and…

Index Range Scans

Optimizer chooses Index Range Scans for selective queries which have bounded as well as unbounded data. Thus for every predicate clause having >,>=,<,<= condition on an indexed column, Oracle optimizer would go for Index Range Scans unless fetching large amount of data or unselective query which favours FTS. Following are the situations where optimizer chooses…

Index Unique Scans

Index Unique Scans returns at most 1 rowid, this means that whenever the condition in WHERE clause guarantees that only one row would be returned Oracle optimizer chooses Index Unique Scans. To summarize this, Index Unique Scan access path is used when all columns of a unique (B-tree) index or an index created as a…

Table Access by Rowid

Rowid is an internal representation of the storage location of data. The rowid of a row specifies the data file and data block containing the row and the location of the row in that block. Locating a row by specifying its rowid is the fastest way to retrieve a single row because it specifies the…