implement：implements full support for DECIMAL(M, D) with M up to 65

[robll-v1]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #23910

What this PR does / why we need it:

This PR implements full support for DECIMAL(M, D) with M up to 65 (backed by the decimal256 type), bringing it from a skeletal type stub to a production-usable numeric type.

What was added across two commits

Commit 1 — d9a6e16: Basic query path

• Parser now accepts DECIMAL(M) up to M=65 and routes M>38 to T_decimal256
• Decimal256 parse/format, float/string/int cast, encode/decode, compare, and vector infrastructure wired up
• CAST('...' AS DECIMAL(65,30)), DDL, INSERT, SELECT, and frontend output working
• BVT case added: test/distributed/cases/dtype/decimal_256.test

Commit 2 — fa7995f: Complete operator and function support

• Arithmetic: +, -, *, /, %, div — scale-aware Add/Sub/Mul/Div/Mod methods added to Decimal256, wired into arithmetic.go and baseTemplate.go
• Aggregation: SUM, AVG via new sumAvgDec256Exec exec path; MIN, MAX via newDecimal256MinMaxExec; return types updated in sumavg2.go
• Math functions: ABS, SIGN, CEIL, FLOOR, ROUND, TRUNCATE — implementations in func_unary.go / func_binary.go, registered in list_builtIn.go
• Predicates: =, !=, <, >, <=, >=, <=>, BETWEEN, IN, NOT IN — valueDec256Compare helper added, all comparison function bodies extended
• Conditional expressions: CASE WHEN, COALESCE, IFF wired in operatorSet.go and list_operator.go
• Sorting: ORDER BY supported via pkg/sort/sort.go
• Type coercion: implicit cast rules in type_check.go extended with full T_decimal256 cross-type entries; list_operator.go retType functions updated for all arithmetic operators

Testing

• Unit tests added/extended: decimal256_support_test.go, sumavg2_test.go, minmax2_test.go, decimal_test.go
• BVT: decimal_256.test — 10/10 pass via mo-tester
• go test passes for: pkg/container/types, pkg/sql/plan/function, pkg/sql/colexec/aggexec, pkg/sort
• make build clean

---

Known Risks and Limitations

1. ZoneMap (ZM) pruning is disabled for decimal256 columns

In pkg/vm/engine/tae/index/zm.go, the UpdateZM and BatchUpdateZM paths skip T_decimal256 entirely. The raw Decimal256 value is 32 bytes, which does not fit the current fixed-layout zonemap encoding. This means:

• Correctness is not affected — queries return correct results
• Performance is degraded on large tables: WHERE decimal256_col > x cannot use min/max block pruning and will fall back to full scan
• This is intentional to avoid a flush-time panic; a proper ZM fix requires a layout change and is deferred

2. No window function support

RANK, ROW_NUMBER, LAG, LEAD, FIRST_VALUE, LAST_VALUE etc. have not been extended for T_decimal256. Using decimal256 columns in window functions will return an unsupported-type error.

3. No bitwise / BIT_AND / BIT_OR / BIT_XOR / OCT support

These are edge-case operations on decimal types in general and are not implemented for decimal256.

4. No LEAST / GREATEST / SERIAL / SERIAL_EXTRACT support

These convenience functions are not wired for T_decimal256 and will return unsupported errors if called with decimal256 arguments.

5. AVG precision cap

AVG return type for decimal256 is capped at scale = min(input_scale + 6, 30) consistent with the existing decimal128 policy. This is a known approximation, not a regression.

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[Copilot]

Pull request overview

This PR upgrades DECIMAL support to fully handle DECIMAL(M,D) up to M=65 by introducing/activating decimal256 across parsing, planning, execution, vector ops, functions/operators, aggregation, sorting, and frontend formatting/export paths (issue #23910).

Changes:

• Extend MySQL parser + planner/binder type selection to route DECIMAL(39..65, D) to T_decimal256.
• Add decimal256 support across casts, comparisons, arithmetic/operators, math functions, SUM/AVG/MIN/MAX, sorting, and vector encode/decode/minmax.
• Add/update unit tests and distributed test expectations for the new type and behaviors.

Reviewed changes

Copilot reviewed 52 out of 52 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/distributed/cases/operator/is_operator.result	Updates expected output (including new result formatting) and removes the previous parser error for DECIMAL(65,30).
test/distributed/cases/function/func_datetime_unixtime.result	Adjusts expected behavior of FROM_UNIXTIME(huge_literal) to return null instead of an error.
test/distributed/cases/dtype/numeric.sql	Updates “out of range” numeric tests to exceed new max precision (66/67).
test/distributed/cases/dtype/numeric.result	Updates expected parser error messages to reflect new 0..65 range.
test/distributed/cases/dtype/decimal_256.test	Adds distributed BVT coverage for decimal(65,30) DDL/DML/functions/aggregations.
test/distributed/cases/dtype/decimal_256.result	Expected results for the new distributed decimal256 test.
test/distributed/cases/dtype/decimal.test	Updates “out of range” decimal tests to exceed new max precision (66/67).
test/distributed/cases/dtype/decimal.result	Updates expected parser error messages to reflect new 0..65 range.
pkg/vm/engine/tae/index/zm.go	Skips zonemap maintenance for decimal256 to avoid layout/flush issues.
pkg/sql/plan/make.go	Enables constant decimal expression typing to fall back to decimal256; adds default rewrite for empty decimal256 type.
pkg/sql/plan/function/type_check.go	Extends type coercion / fixed cast rules to include decimal256 and scale propagation rules.
pkg/sql/plan/function/operator_in.go	Adds decimal256 to generic IN/NOT IN operator type set.
pkg/sql/plan/function/operator_between.go	Adds BETWEEN / in-range support using CompareDecimal256.
pkg/sql/plan/function/operatorSet.go	Wires decimal256 into CASE/IFF/COALESCE and supported type sets; adds unary minus impl.
pkg/sql/plan/function/list_operator.go	Registers operator overloads/return-type rules for decimal256 (IN/NOT IN, arithmetic, unary, coalesce).
pkg/sql/plan/function/list_builtIn.go	Registers math built-ins and FROM_UNIXTIME overloads for decimal256.
pkg/sql/plan/function/list_agg.go	Enables decimal256 for SUM and MIN/MAX supported types.
pkg/sql/plan/function/func_unary.go	Implements ABS and SIGN for decimal256.
pkg/sql/plan/function/func_testcase.go	Extends function test harness to compare/build vectors for decimal256.
pkg/sql/plan/function/func_compare.go	Adds compare operator support and valueDec256Compare to handle scale alignment.
pkg/sql/plan/function/func_cast_test.go	Adds unit tests for casting to decimal256 from string and int64.
pkg/sql/plan/function/func_cast.go	Adds decimal256 casting support (to/from numeric/string/binary-ish types) and routing in NewCast.
pkg/sql/plan/function/func_binary_test.go	Adds FROM_UNIXTIME(decimal256) unit test.
pkg/sql/plan/function/func_binary.go	Adds decimal256 math funcs (ceil/floor/round/truncate) and FROM_UNIXTIME(decimal256) implementations.
pkg/sql/plan/function/decimal256_support_test.go	Adds broad unit coverage for decimal256 operators, coercion, arithmetic, conditional ops, math, etc.
pkg/sql/plan/function/baseTemplate.go	Extends decimal arithmetic template to include decimal256 and zero-checks.
pkg/sql/plan/function/arithmetic.go	Adds decimal256 operator support (plus/minus/mul/div/mod/div) and result-type driven handling.
pkg/sql/plan/build_util.go	Maps AST decimal types with precision > 38 to T_decimal256.
pkg/sql/plan/base_binder.go	Updates numeric literal binding to fall back to decimal256 parsing when decimal128 parsing fails; generalizes decimal check for floats.
pkg/sql/parsers/dialect/mysql/mysql_sql_test.go	Adds parser test for decimal(65,30).
pkg/sql/parsers/dialect/mysql/mysql_sql.y	Extends grammar to accept decimal precision up to 65 and to build appropriate internal type.
pkg/sql/parsers/dialect/mysql/mysql_sql.go	Regenerated parser output matching mysql_sql.y changes.
pkg/sql/colexec/aggexec/sumavg2_test.go	Extends SUM/AVG tests to include decimal256 vectors and validation.
pkg/sql/colexec/aggexec/sumavg2.go	Adds SUM/AVG execution support for decimal256 (including return type rules).
pkg/sql/colexec/aggexec/minmax2_test.go	Adds unit tests for MIN/MAX over decimal256.
pkg/sql/colexec/aggexec/minmax2.go	Adds MIN/MAX exec path for decimal256.
pkg/sort/sort_test.go	Adds unit test for sorting decimal256.
pkg/sort/sort.go	Adds decimal256 sort support.
pkg/frontend/util.go	Enables scalar expr value extraction for decimal256.
pkg/frontend/output.go	Adds decimal256 formatting for result output and column slice conversion.
pkg/frontend/mysql_cmd_executor.go	Maps engine decimal256 to MySQL DECIMAL type for column metadata.
pkg/frontend/export_parquet.go	Adds decimal256 conversion for Parquet export values.
pkg/frontend/export.go	Adds decimal256 formatting for export and JSON conversion.
pkg/container/vector/vector.go	Adds core vector operations for decimal256 (GetAny/Shrink/Shuffle/UnionAll/const set/String/minmax/sort/compact).
pkg/container/vector/tools.go	Adds append-bytes function support for decimal256.
pkg/container/vector/functionTools.go	Adds function result wrapper support for decimal256.
pkg/container/types/types.go	Extends DecimalWithFormat to include Decimal256.
pkg/container/types/encoding.go	Adds encode/decode and size constants for decimal256 values.
pkg/container/types/decimal_test.go	Adds parse/format test for decimal256 and negative-scale float conversion test.
pkg/container/types/decimal.go	Implements substantial decimal256 parsing/formatting, arithmetic helpers, trunc-scaling, float conversions, etc.
pkg/container/types/compare.go	Adds decimal256 asc/desc compare helpers.
pkg/compare/compare.go	Adds comparator construction + copy function for decimal256.

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[robll-v1]

I fixed the clear issue-level gaps in this round:

1. decimal256 cast support
   I added the missing decimal256 source casts for bit/int/uint, so decimal256 is no longer missing the basic
   integral cast paths that other decimal types already expose.

2. silent scale-overflow handling in decimal256 comparison
   I changed the decimal256 compare path to return the scale overflow error explicitly instead of ignoring the
   Scale() error and continuing with a potentially wrong comparison result.

I also added focused tests for both changes.

ZoneMap bypass for decimal256 is a real performance limitation, but I did not address it in this patch. The
current zonemap raw layout cannot safely store 32-byte decimal256 values, so fixing it cleanly needs a
storage/index layout change rather than a small function-level patch.

The cast-gap report is only partially applicable now. I fixed the missing decimal256 integral cast paths
(bit/int/uint). I did not extend decimal256 to date/datetime/time/timestamp/year in this patch, because
that would widen the temporal cast surface beyond the scoped fix and is better handled as a follow-up
behavior discussion.

Decimal256 Format() using big.Int is a valid performance concern. I agree it is worth optimizing, but I
treated it as a follow-up perf improvement rather than a correctness blocker for issue 23910.

I do not see evidence that Decimal128.Mod128 behavior changed in this branch. The branch adds decimal256-
specific helpers, but Decimal128.Mod128 itself was not changed here.

FROM_UNIXTIME behavior did change for very large numeric literals after decimal256 widened the literal
typing path. I left that behavior as-is for now because it is a compatibility discussion, not a clear
correctness bug in the decimal256 execution path itself.

There is still duplicated decimal256 code across the decimal64/128/256 paths. That is real technical debt,
but I kept this patch scoped to correctness/compatibility fixes instead of refactoring the whole decimal
family.

Some result-file churn in this branch is broader than the core decimal256 behavior change. I agree that is
test hygiene noise, but it does not affect runtime correctness.