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Error in Lua script (ZUNIONSTORE + unpack(KEYS) + AGGREGATE MIN) - Redis 2.6
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vicmargar  
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 More options Apr 27 2012, 9:50 pm
From: vicmargar <vicmar...@gmail.com>
Date: Fri, 27 Apr 2012 18:50:24 -0700 (PDT)
Local: Fri, Apr 27 2012 9:50 pm
Subject: Error in Lua script (ZUNIONSTORE + unpack(KEYS) + AGGREGATE MIN) - Redis 2.6
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Hello, I've been trying to implement a use case for Redis sorted sets
using Lua scripting in Redis 2.6 and after a lot of time without being
able to make it work I've managed to reproduce my problem in a simple
scenario. I'm still not sure if I'm doing something wrong but
basically I can't get ZUNIONSTORE to work if i'm using unpack and
'AGGREGATE' 'MIN' at the same time:

redis 127.0.0.1:6379> zadd set1 1 key1
(integer) 1
redis 127.0.0.1:6379> zadd set1 1 key2
(integer) 1
redis 127.0.0.1:6379> zadd set2 2 key1
(integer) 1
redis 127.0.0.1:6379> zadd set2 1 key3
(integer) 1
redis 127.0.0.1:6379> eval "return redis.call('zunionstore', 'tmp', 3,
unpack(KEYS))" 3 set1 set2 set3
(integer) 3
redis 127.0.0.1:6379> zrange tmp 0 -1 WITHSCORES
1) "key2"
2) "1"
3) "key3"
4) "1"
5) "key1"
6) "3"
redis 127.0.0.1:6379> eval "return redis.call('zunionstore', 'tmp', 3,
unpack(KEYS), 'AGGREGATE', 'MIN')" 3 set1 set2 set3
(integer) 2
redis 127.0.0.1:6379> zrange tmp 0 -1 WITHSCORES
1) "key1"
2) "1"
3) "key2"
4) "1"
redis 127.0.0.1:6379> eval "return redis.call('zunionstore', 'tmp', 3,
'set1', 'set2', 'set3', 'AGGREGATE', 'MIN')" 0
(integer) 3
redis 127.0.0.1:6379> zrange tmp 0 -1 WITHSCORES
1) "key1"
2) "1"
3) "key2"
4) "1"
5) "key3"
6) "1"

Thank you for your help!


 
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Ziyan Zhou  
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 More options Jul 22 2012, 7:14 pm
From: Ziyan Zhou <ziyan....@gmail.com>
Date: Sun, 22 Jul 2012 16:14:46 -0700 (PDT)
Local: Sun, Jul 22 2012 7:14 pm
Subject: Re: Error in Lua script (ZUNIONSTORE + unpack(KEYS) + AGGREGATE MIN) - Redis 2.6
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Just ran into this issue too.

Your problem lies in upack(KEYS).

Your "return redis.call('zunionstore', 'tmp', 3, unpack(KEYS), 'AGGREGATE',
'MIN')" is actually equivalent to "return redis.call('zunionstore', 'tmp',
3, KEYS[1], 'AGGREGATE', 'MIN')"

In general if you do
local a = {100, 200, 300}

{1, unpack(a), 2} will give you {1, 100, 2}

My solution:

local args = {'zunionstore', 'tmp', 3, unpack(KEYS)}
args[#args + 1] = 'AGGREGATE'
args[#args + 1] = 'MIN'
redis.call(unpack(args))

I am not sure why lua is so retarded.


 
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Javier Guerra Giraldez  
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 More options Jul 23 2012, 10:20 am
From: Javier Guerra Giraldez <jav...@guerrag.com>
Date: Mon, 23 Jul 2012 09:20:46 -0500
Local: Mon, Jul 23 2012 10:20 am
Subject: Re: Error in Lua script (ZUNIONSTORE + unpack(KEYS) + AGGREGATE MIN) - Redis 2.6
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On Sun, Jul 22, 2012 at 6:14 PM, Ziyan Zhou <ziyan....@gmail.com> wrote:
> I am not sure why lua is so retarded.

to keep consistency to the arguments list.

the (fixed) 'AGGREGATE' and 'MIN' arguments are guaranteed to stay at
the same position, that is fifth and sixth.

multiple-returns are only allowed to expand fully at the end of an
argument list, so that subsequent arguments are not pushed around
unpredictably

--
Javier


 
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Ziyan Zhou  
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 More options Jul 24 2012, 4:57 pm
From: Ziyan Zhou <ziyan....@gmail.com>
Date: Tue, 24 Jul 2012 13:57:52 -0700 (PDT)
Local: Tues, Jul 24 2012 4:57 pm
Subject: Re: Error in Lua script (ZUNIONSTORE + unpack(KEYS) + AGGREGATE MIN) - Redis 2.6
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Problem is there is no clear indication of what went wrong, it is hard to
debug especially in vicmargar's case where the number of arguments stayed
the same. His bug caused strange outcome, yet there is nothing to indicate
an error, just hard to debug in general.

Also the code can be changed to:

local args = {'zunionstore', 'tmp', #KEYS, unpack(KEYS)}
args[#args + 1] = 'AGGREGATE'
args[#args + 1] = 'MIN'
redis.call(unpack(args))

To accommodate any number of KEYS, so 'AGGREGATE' and 'MIN' are not
guaranteed to stay at 5th or 6th, instead, they are guaranteed to stay at
the end.


 
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Javier Guerra Giraldez  
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 More options Jul 24 2012, 5:07 pm
From: Javier Guerra Giraldez <jav...@guerrag.com>
Date: Tue, 24 Jul 2012 16:07:36 -0500
Local: Tues, Jul 24 2012 5:07 pm
Subject: Re: Error in Lua script (ZUNIONSTORE + unpack(KEYS) + AGGREGATE MIN) - Redis 2.6
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On Tue, Jul 24, 2012 at 3:57 PM, Ziyan Zhou <ziyan....@gmail.com> wrote:
> Also the code can be changed to:

> local args = {'zunionstore', 'tmp', #KEYS, unpack(KEYS)}
> args[#args + 1] = 'AGGREGATE'
> args[#args + 1] = 'MIN'
> redis.call(unpack(args))

sure, that's the way to do it

> To accommodate any number of KEYS, so 'AGGREGATE' and 'MIN' are not
> guaranteed to stay at 5th or 6th, instead, they are guaranteed to stay at
> the end.

this is appropriate for a command line, or a data description language
(like Redis commands); but for a programming language, functions tend
to pick arguments at fixed positions in the arguments list, and that's
why Lua is designed like this.

--
Javier


 
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