How Large will a Table be

[Steve]

Wondering if there is some function of good method of estimating how much space a live table with varying types, and a varying number of rows will take once you save it.

Is there a way to determine how much memory a table or object is currently using?
So we can get an idea of what it’s size will be when written to disk.

[Ryan Hamilton]

-22! can be used to get the size of a table in memory, which will be very close to the on-disk space needed:
q)-22!([] a:til 1000)
8031
q)-22!([] a:til 2000)
16031
q)`:/temp/a set ([] a:til 2000) / save to file
`:/temp/a
/ hcount returns size of file on disk
q)hcount `:/temp/a
16025

For the larger question of estimating database requirements:

Notice in the data types table there’s a size column:
http://www.timestored.com/kdb-guides/kdb-reference-card

To estimate storage requirements:
Design your tables and what columns they contain.
Then for each table:
1. Multiply the size in bytes for each type to get the number of bytes needed per row.
2. Estimate the number of rows you will have now? 1 year from now? 2 years from now? As you want to be careful not just of the data size now but of of data growth rates.
3. Multiply the estimated size of a row by the number of rows forecast. To get the space requirements for that one table.
4. Total the values of all tables (and multiply by 1.5 for safety 🙂

[Author]

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