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Re: How to estimate memory usage ?
Posted by: Ingo Strüwing
Date: December 21, 2006 01:47PM

Hi.

CREATE TABLE `t3` (
`c1` int(11) DEFAULT NULL,
`c2` char(20) DEFAULT NULL,
`c3` char(8) DEFAULT NULL,
KEY `c1` (`c1`),
KEY `c2` (`c2`) USING BTREE,
KEY `c3` (`c3`)
) ENGINE=MEMORY;

Assuming 32-bit machine: sizeof(char*) == 4.
One btree index, key length 20.
Two hash indexes.
Row length 33.

SUM_OVER_ALL_BTREE_KEYS(max_length_of_key + sizeof(char*) × 4)
+ SUM_OVER_ALL_HASH_KEYS(sizeof(char*) × 2)
+ ALIGN(length_of_row+1, sizeof(char*)):

(20 + 4 * 4)
+ 2 * (4 * 2)
+ ALIGN(33 + 1, 4):

(36)
+ 16
+ 36 (34 rounded up to the next multiple of 4):

88 bytes per row. That's what the formula says.

Regards
Ingo

Ingo Strüwing, Senior Software Developer - Storage Engines
MySQL AB, www.mysql.com

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