Debian starts PostgreSQL at boot time. Is this necessary or can I remove it?

I know I could experiment but I don't want to break anything.

Debian has a couple different tools you can use for adjusting What services start in what run levels. rcconf a basic ON/OFF tool, and sysv-rc-conf Which provides more granular control.
Installing one of these will allow you to easily set what services start in particular run levels.

PostgreSQL should not be necessary unless you are using it for a specific task.

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apt-get install sysv-rc-conf
sysv-rc-conf --level 2345 postgresql off
That command should set postgres to not start at boot time.

If you need to re-enable it, you can simply
sysv-rc-conf --level 2345 postgresql on

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alternately you could use rcconf

apt-get install rcconf
rcconf


then simply use the arrow keys to select the service you want to change startup on and toggle it on or off..

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or you could make the change manually without using any of the config tools....

mv /etc/rc2.d/S20postgresql /etc/rc2.d/K20postgresql

S means Start K means Kill

That seems like the manly-man way to do it, but I opted for rcconf ... works great.

I was surprised to note that alsa was not selected to start at boot, but my sound works fine so I left it unenabled.