feature: dynamic-precision decimal type

[fuyufjh]

In PostgresQL, there are 2 kinds of decimal (or numeric) types:

• NUMERIC(precision, scale) means all data in such columns are of specified precision & scale. In this case we can only store a 'flatten' integer in array or storage because the position of dot is fixed.
• NUMERIC means data in such columns are with variable precision & scale. In such case, precision & scale must be kept along with the 'flatten' integer.

Currently, we only support fixed-point decimals, but considering that we are going to support schemaless in the future, it's better to support dynamic-precision decimals.

Order-preserved encoding of dynamic-precision decimals is difficult. I would recommend to investigate on cockroachdb first before implementation.

References:

0. PostgreSQL: Documentation: 9.1: Numeric Types
1. cockroach/decimal.go · cockroachdb/cockroach
2. SQLite4: Key Encoding

[lmatz]

singularity-data/risingwave-legacy#2556 related

[lmatz]

Another reference is tidb It seems all of them are quite complicated. :sweat_smile: