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Estimate spaced usage for new Oracle index

Oracle Database Tips by Donald BurlesonApril 30, 2015

Question:  I want to be able to accurately estimate the amount of disk space used by an Oracle index.  I assume that the PCTFREE=10 and I know the blocksize and average key length, but I wonder if Oracle has a routine that will estimate the amount of space consumed by a new index?

Answer: The spaced used by an index depends on several factors:

  • Blocksize:  You can use between a 2k to 32k blocksize and there is a slight variation in space usage between index blocksizes.
  • Key Length:  The size of the index key will obviously effect the size of the resulting index.
  • PCTFREE: The amount of space left for index row expansion

You can use the Oracle capacity planning spreadsheets to estimate table and index size, and this formula estimates the space requirements for a single Oracle index.

You can also invoke the dbms_metadata.get_ddl procedure to estimate the current size of your indexes:

Franck Pachot has a method where you can use the plan table to estimate the size of all indexes with dbms_metadata. The PL/SQL below loops though all indexes in user_indexes and uses the get_ddl procedure to gather the total estimated space used for all indexes in the schema:

for r in ( select index_name from user_indexes ) loop
execute immediate 'explain plan set statement_id='''||r.index_name||''' for '||
end loop;

cast(extractvalue(xmltype(other_xml),'/other_xml/info[@type="index_size"]') as number) estimated_index_size
plan_table p

You can use the dbms_space package create_index_cost [procedure to estimate the size of an index.  The dbms_space.create_index_cost accepts the schema owner, the "create index statement" (which includes the table and key names, and the amount of space used and allocated.

used- The number of bytes representing the actual index data.

alloc - Size of the index when created in the tablespace.

SQL> variable used number
SQL> variable alloc number
SQL> set autoprint on

SQL> exec dbms_space.create_index_cost('create index scott.idx_test on scott.emp (emp_id, emp_hire_date) LOCAL', :used, :alloc );

For global sizing estimate of tables and indexes, I recommend the Oracle capacity planning spreadsheets. To estimate the size of an index for the storage clause, you can use the following procedure:

1. Calculate the required database block header size (BHS).

BHS = fixed header + variable transaction header

fixed header = 113
variable transaction header = 24 *INITRANS

2. Calculate available data space (ADS).
ADS = ((Blocksize - BHS) * (PCTFREE/100))

3. Calculate the average data length (ADL) per row.

Note: Step 3 is the same as step 3 in the table-sizing section in Chapter 5. However, size only for those columns in the index.

4. Calculate the average row length (ARL).
bytes/entry = entry header + ROWID length + F + V + D

entry header = 2

F = Total length of bytes of all columns that store 127 bytes or fewer—one header byte per column.

V = Total length of bytes of all columns that store more than 127 bytes—two header bytes per column.

For UNIQUE indexes, the entry header is 0.

5. Calculate number of blocks for index.

# of Blocks = 1.05 * (((# of NOT NULL rows) / (ADS) / ARL))

The 1.05 factor allows for branch space and is an empirically (SWAG) derived value.

6. Calculate the number of bytes required.
Size in bytes = BLOCKSIZE - number of blocks

Also see measuring average rows per block in an Oracle index.

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