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Don Burleson Blog 







Oracle library cache SQL hash value

Oracle Database Tips by Donald BurlesonAugust 22, 2015

Question:  When and Where HASH VALUE for a given SQL statement get generated?  I need to understand how hashing works within the library cache and how Oracle implements hashing.

Answer:  When a SQL statement first enters Oracle, several steps take place:

1 - The SQL is standardized in capitalization, so upper and lower case do not hash to a different value

2 - Oracle then feeds this SQL into their hashing algorithm to generate a unique RAM address in the library cache.  The hash is built to avoid collisions (two statements generating to the same address) and it will always find a space within the library cache.  The hashing algorithm is a proprietary secret, but it works very fast, using a SQL statement to generate a RAM address in just a few microseconds.

3 - Oracle then goes to the location indicated by the hash and if the statement already exists, Oracle will re-use the SQL statement, a "soft" parse.  Else, the SQL statement is loaded into this RAM location, and the SQL statement is processed as a "hard" parse.

Also see my notes on hashing of SQL statements for more details on the internals of Oracle hashing of SQL.

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