Hints in Oracle

•Hints are the special command in oracle which instruct oracle database optimizer to choose an execution plan for a statement and is specified within comment inside a sql statement.

•The comment containing hints must follow the SELECT, UPDATE, INSERT, MERGE, or DELETE keyword and only one hint is allowed in a statement block.

•Only two hints are used with INSERT statements: The APPEND hint always follows the INSERT keyword, and the PARALLEL hint can follow the INSERT keyword.

•The syntax of hint in a sql statement holds the following format.

INSERT, UPDATE, DELETE, SELECT, or MERGE keyword plus /*+ plus hint name plus string plus */

or,
INSERT, UPDATE, DELETE, SELECT, or MERGE keyword plus --+ plus hint name plus string

An example is,

SELECT /*+ FULL(e) */

Note that the plus sign (+) causes Oracle to interpret the comment as a list of hints. Note that after delimiter * plus(+) sign should follow immediately. No space is permitted.

The --+ syntax requires that the entire comment be on a single line.

The available hints will be discussed in another topic.

While dealing with oracle hints we have to remember several things.
Oracle Database ignores hints and does not return an error under the following circumstances:

•The hint has misspellings or syntax errors. However, the database does consider other correctly specified hints in the same comment.

•If hints containing comments does not appear just after DELETE, INSERT, MERGE, SELECT, or UPDATE keyword.

•Hints conflict with one another. However, the database does consider other hints in the same comment.
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