Oracle SQL is parsed before execution, and
checked for syntax (and parts of the semantic check) before the SQL is
loaded into the library cache. As opposed to a soft parse (which
does not require loading into the shared pool), a hard
parse includes these steps:
shared pool - The SQL source code is loaded into RAM for
parsing. (the "hard" parse step)
Syntax parse - Oracle parses the
syntax to check for misspelled SQL keywords.
Semantic parse - Oracle verifies all
table & column names from the dictionary and checks to see if
you are authorized to see the data.
Query Transformation - Oracle will transform
complex SQL into simpler, equivalent forms and replace
aggregations with materialized views, as appropriate. In earlier
releases of Oracle the query_rewrite=true parameter had
to be set for materialized view rewriting.
Optimization - Oracle then creates
an execution plan, based on your schema statistics (or maybe
with statistics from dynamic sampling in 10g). Oracle
build the decision tree of costs during this period, choosing
the path with the lowest perceived cost.
Create executable - Oracle builds an
executable file with native file calls to service the SQL query.
Fetch rows - Oracle then
executes the native calls to the data files to retrieve the rows
and passes them back to the calling program.
Anytime a session issues SQL statement that
does not already exist in the shared pool, then Oracle has to do
a hard parse. essentially performing all of the above steps.
If the statement already exists, then a soft parse occurs,
skipping step 1.
In a soft parse, Oracle must still perform a
syntax parse and semantic check because it is possible that a
DDL change altered one of the target tables or views since the
SQL statement was originally executed. In the case of DDL,
all related SQL is marked as invalidated.
Oracle gives us the
shared_pool_size parm to cache SQL so that we don't have to
parse, over-and-over again. However, SQL can age-out if
the shared_pool_size is too small or if it is cluttered with
non-reusable SQL (i.e. SQL that has literals "where name =
"fred") in the source. Hence, the
shared_pool_size parameter (memory_target in 12c
AMM) has an impact of hard parses, because re-entrant SQL can
age out of the shared pool.
What the difference between a hard
parse and a soft parse in Oracle? Just the first step,
step 1 as shown in red, above. In other words, a soft
parse does not require a shared pool reload (and the associated
RAM memory allocation).
general high "parse call" (> 10/sec.) indicates that your system
has many incoming unique SQL statements, or that your SQL is not
reentrant (i.e. not using bind variables).
A hard parse is when
SQL must be re-loaded into the shared pool.
A hard parse is worse than a soft parse because of the overhead
involved in shared pool RAM allocation and memory management.
Once loaded, the SQL must then be completely re-checked for
syntax & semantics and an executable generated.
Excessive hard parsing can occur when your shared_pool_size
is too small (and reentrant SQL is paged out), or when you have
non-reusable SQL statements without host variables.
See the cursor_sharing
parameter for a easy way to make SQL reentrant and remember that
you should always use host variables in you SQL so that they can
Get Complete Oracle Tuning Details
The landmark book
Tuning: The Definitive Reference Third Edition" has been
updated with over 800 pages of expert performance tuning tips.
It's packed with scripts and tools to hypercharge Oracle performance and you can
buy it for 40% off directly from the publisher.