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| Re: how to calculate the storage size of table and index [message #20973 is a reply to message #20961] |
Wed, 03 July 2002 23:25   |
Satish Shrikhande
Messages: 167
Registered: October 2001
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Senior Member |
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Suppose, you have to create a new table and insert some data into it, or table already exists and you will make inserts. You want to estimate the final size of your table, for example you want to set correct storage parameters.
You can do it in two ways:
Using internal statistics.
Math estimation.
It's very rough estimation.
I-st method - using statistics
Analyze statitics for the table.
From SQLPlus run the following command:
ANALYZE TABLE
COMPUTE STATISTICS;
or
ANALYZE TABLE
ESTIMATE STATISTICS;
Make select form DBA_TABLES, ALL_TABLES or USER_TABLES dictionary view.
SELECT avg_row_len
FROM dba_tables
WHERE table_name = '
';
where
avg_row_len - is an average row length (in bytes), including row overhead;
<ESTIMATED_TABLE_SIZE> = avg_row_len * number_of_rows * (1 + PCTFREE/100) * 1.15
where number_of_rows - expected number of rows in you table;
1.15 - just in case :).
NOTE: If it's a new table or it's empty, you may insert some "demo" records before analyzing the table.
II-nd method - using math estimation
Calculate maximum row length of the table based on the following facts:
CHAR( n ), VARCHAR2( n ) - n bytes
DATA - 7 bytes;
NUMBER( p, s ) = floor( (p+1)/2 ) + 1, if it's a negative number - add one byte (+1).
For example, Oracle uses 3 bytes to store number: 512 (NUMBER(3)).
<ESTIMATED_TABLE_SIZE> =
MAX_ROW_LENGTH * (1 + PCTFREE/100) * number_of_rows * 1.15
where 1.15 - just in case :).
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