How to delete ETS records older than xxx?

Questions / Help
1

So I’ve got an ETS table, I have some records there. One of the values (terms) in every record is an equivalent of “updated_at”. I’d like to delete all “dormant” records, where the said field contains value lower than a predefined difference to current time in the same format. Something like WHERE <condition> in SQL I got to
https://www.erlang.org/doc/man/ets.html#match_delete-2
and also to:
https://www.erlang.org/doc/apps/erts/match_spec.html#ets-examples
but if some ETS experts out there are able to put a clue out of their heads that would make life more enjoyable again :wink:

2

How and at what position do you store updated_at? What type is it?

Erlang -- ets might be useful to construct a match statement, but note the warning.

# use fun2ms to get a general idea of what your match spec would look like
# here I match a record with four fields where third field is less than 100 (doesn't matter)
# and I select a constant 1
iex(4)> :ets.fun2ms fn {_, _, updated_at, _} when updated_at < 100 -> 1 end
[{{:_, :_, :"$1", :_}, [{:<, :"$1", 100}], [1]}]

iex(5)> tab = :ets.new(:tab, [])
#Reference<0.2610923153.3860463617.223839>

# I insert a record with some int (representing current time) as the third record
iex(7)> :ets.insert(tab, {"something", "something", _updated_at = System.os_time, "something"})
true

# I select the third element from records that have four elements and the third element is more than 100
iex(8)> :ets.select(tab, [{{:_, :_, :"$1", :_}, [{:>, :"$1", 100}], [:"$1"]}])
[1637256726836072000]

# same spec, but used for deletion
iex(9)> :ets.select_delete(tab, [{{:_, :_, :"$1", :_}, [{:>, :"$1", 100}], [true]}])
1

iex(10)> :ets.tab2list(tab)
[]
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3

It’s an int (UNIX seconds) stored at third position (those are for some reasons indexed from one rather than zero, right?) so it would be position “3”.

Thanks. OK - so it’s a select rather than match. Do I understand correctly that match_delete/2 would not be able to cut it?

4

Yes, :ets.match can only match a record, that is it can check for equality/pattern of some fields, but not do guard-like checks: Erlang -- ets.

5

That’s not correct. match_spec’s can have guards, see: GitHub - ericmj/ex2ms: :ets.fun2ms for Elixir, translate functions to match specifications

6

Are you sure we’re talking the same thing? I found the match_spec documentation:
https://www.erlang.org/doc/apps/erts/match_spec.html

but (putting aside my getting around the match_spec stuff) I wasn’t able to get match_delete/2 to do what I wanted it to do. The select_delete/2 seems to work though. So if you say that match_delete/2 can do the same then I am puzzled. But if you’re saying that the match spec in general can have guards then it’s all OK but @ruslandoga was answering my question about match_delete/2

7

Oh. I didn’t realize the comment was specifically on match/match_delete. Yes, only match_spec supporting api has guards and such.

8

Another “interesting” thing is that the already mentioned Erlang docs
https://www.erlang.org/doc/apps/erts/match_spec.html#ets-examples

give their examples with single quotes, while the fun2ms – existence of which I was not aware until @ruslandoga’s excellent answer (he even guessed my record arity plus type and position of the field in question!) – returns everything “atomised”. Now, what am I missing this time, which cost me a portion of my life today? IOW - how was I supposed to know that I have to rewrite those examples using “atomised” equivalents? I am guessing this has a lot to do with Elixir / iex treating of charlists vs. what “pure” Erlang does with them?

9

Yes, You need to translate some syntaxes when using Erlang’s lib.

10

Yes, the single quotes in Erlang are atoms so the Erlang '$1’ is the same as Elixir :"$1" .

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