Inconsistent data types in SQL resultset when using SUM function with a subquery

Bug description

Calculating SUM of an integral subquery column returns 'decimal' column header with 'number' values.

• OS: Linux
• OS Version: Centos 8
• Architecture: amd64

Tarantool 2.10.0-beta2-286-gb494e762e
Target: Linux-x86_64-Release
Build options: cmake . -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_BACKTRACE=ON
Compiler: /usr/lib64/ccache/cc /usr/lib64/ccache/c++
C_FLAGS: -fexceptions -funwind-tables -fno-common -fopenmp -msse2 -std=c11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -Wno-gnu-alignof-expression -fno-gnu89-inline -Wno-cast-function-type
CXX_FLAGS: -fexceptions -funwind-tables -fno-common -fopenmp -msse2 -std=c++11 -Wall -Wextra -Wno-strict-aliasing -Wno-char-subscripts -Wno-format-truncation -Wno-invalid-offsetof -Wno-gnu-alignof-expression -Wno-cast-function-type

Steps to reproduce

Run tarantool. Execute the following script:

box.cfg{}

test1 = box.schema.create_space("test1")

test1:format({{ name = "id", type = "unsigned" }})

test1:create_index("primary")

test1:insert({1})
test1:insert({2})
test1:insert({3})

Now run the following query:

tarantool> box.execute([[select "id" from "test1"]])
---
- metadata:
  - name: id
    type: unsigned
  rows:
  - [1]
  - [2]
  - [3]
...

Note the type of the column. It's 'unsigned'. Everything is OK so far. Run this:

tarantool> box.execute([[select sum("id") from "test1";]])
---
- metadata:
  - name: COLUMN_1
    type: integer
  rows:
  - [6]
...

The type of the column is 'integer' now. It makes me a bit nervous already, but still is not critical. Finally run this:

tarantool> box.execute([[
         >     select sum("id")
         >     from (select "id" from "test1");
         > ]])
---
- metadata:
  - name: COLUMN_1
    type: decimal
  rows:
  - [6]
...

This is a real crash obviously!

1. Why is the type of the column 'decimal' now?
2. What's worse, the values of the column are not really decimal, they have type 'number'!

tarantool> type(box.execute([[
    select sum("id")
    from (select "id" from "test1");
]]).rows[1][1])
---
- number
...

Actual behavior

tarantool> box.execute([[select sum("id") from "test1";]])
---
- metadata:
  - name: COLUMN_1
    type: integer
  rows:
  - [6]
...

tarantool> box.execute([[
         >     select sum("id")
         >     from (select "id" from "test1");
         > ]])
---
- metadata:
  - name: COLUMN_1
    type: decimal
  rows:
  - [6]
...

Expected behavior

tarantool> box.execute([[select sum("id") from "test1";]])
---
- metadata:
  - name: COLUMN_1
    type: unsigned
  rows:
  - [6]
...

tarantool> box.execute([[
         >     select sum("id")
         >     from (select "id" from "test1");
         > ]])
---
- metadata:
  - name: COLUMN_1
    type: unsigned
  rows:
  - [6]
...