aggfuncs: implement avg(distinct) functions

[zz-jason]

What have you changed? (mandatory)

Implement AVG functions which handles the distinct property:

• use map[types.MyDecimal]bool to check whether a decimal values already exists
• use map[float64]bool for float64 values.

What are the type of the changes (mandatory)?

• Improvement (non-breaking change which is an improvement to an existing feature)

How has this PR been tested (mandatory)?

-unit test
-explain test

Refer to a related PR or issue link (optional)

to #6952

[zz-jason]

/run-all-tests

[jackysp]

Please fix CI.

[zz-jason]

/run-all-tests tidb-test=pr/569

[zz-jason]

/run-all-tests tidb-test=pr/569

[zz-jason]

@XuHuaiyu @jackysp PTAL

[winoros]

map[types.MyDecimal]struct{}?

[winoros]

ditto

[zz-jason]

map[types.MyDecimal]bool is much convenient to use and will not introduce much memory overhead.

[zz-jason]

Here is an experiment:

• map[int]bool with 10M elements: 315M
• map[int]struct{} with 10M elements: 286M

1. memory usage of map[int]bool

package main

import (
        "fmt"
        "runtime"
)

func main() {
        numElements := 10000000
        var m1, m2 runtime.MemStats

        runtime.ReadMemStats(&m1)
        a := make(map[int]bool)
        for i := 0; i < numElements; i++ {
                a[i] = true
        }
        runtime.ReadMemStats(&m2)
        fmt.Println("Bytes allocated for A:", m2.Alloc-m1.Alloc)
}

The result is:

[jianzhang.zj:/tmp]
➜ go build a.go

[jianzhang.zj:/tmp]
➜ ./a
Bytes allocated for A: 315457888

2. memory usage of map[int]struct{}

package main

import (
        "fmt"
        "runtime"
)

func main() {
        numElements := 10000000
        var m1, m2 runtime.MemStats

        runtime.ReadMemStats(&m1)
        b := make(map[int]struct{})
        for i := 0; i < numElements; i++ {
                b[i] = struct{}{}
        }
        runtime.ReadMemStats(&m2)
        fmt.Println("Bytes allocated for B:", m2.Alloc-m1.Alloc)
}

the result is:

[jianzhang.zj:/tmp]
➜ go build b.go

[jianzhang.zj:/tmp]
➜ ./b
Bytes allocated for B: 286228080

[winoros]

lgtm

[XuHuaiyu]

LGTM

Using structure{} is the official practice in go

[zz-jason]

/run-all-tests tidb-test=pr/569