Override coalesce with a Doltgres-specific implementation

[fulghum]

The coalesce implementation in GMS does not properly handle Doltgres types. This PR adds a small Doltgres-specific implementation that properly handles Doltgres types.

Fixes: #2332

[github-actions]

	Main	PR
covering_index_scan_postgres	1949.34/s	1928.52/s	-1.1%
groupby_scan_postgres	130.84/s	132.37/s	+1.1%
index_join_postgres	676.14/s	676.37/s	0.0%
index_join_scan_postgres	857.32/s	858.50/s	+0.1%
index_scan_postgres	25.92/s	25.63/s	-1.2%
oltp_delete_insert_postgres	877.45/s	847.32/s	-3.5%
oltp_insert	738.80/s	750.76/s	+1.6%
oltp_point_select	3034.38/s	3071.72/s	+1.2%
oltp_read_only	3044.66/s	3065.29/s	+0.6%
oltp_read_write	2314.39/s	2325.59/s	+0.4%
oltp_update_index	765.43/s	764.29/s	-0.2%
oltp_update_non_index	812.36/s	825.73/s	+1.6%
oltp_write_only	1793.90/s	1791.18/s	-0.2%
select_random_points	1914.13/s	1946.77/s	+1.7%
select_random_ranges	1092.26/s	1132.30/s	+3.6%
table_scan_postgres	24.62/s	24.56/s	-0.3%
types_delete_insert_postgres	811.31/s	809.96/s	-0.2%
types_table_scan_postgres	8.71/s	8.68/s	-0.4%

[itoqa]

Commit: 4eecb70: 10 test cases ran, 1 failed ❌, 8 passed ✅, 1 additional finding ⚠️.

Summary

This run focused on database expression behavior around null-fallback logic in reads and writes, including normal flows, type-edge cases, and retry/atomicity behavior after conversion errors. Overall health is mostly stable, with core fallback semantics and consistency checks behaving as expected across varied query shapes.

Safe to merge — the only regression tied to this PR is a minor issue in a narrow no-op update typing path, while the broader behavior exercised here remained stable. A separate medium-severity update typing failure was also observed but appears pre-existing, so it is a caveat to track rather than a blocker for this merge.

Tests run by Ito

Result	Severity	Type	Description
❌		Flow	The UPDATE assignment should keep val unchanged, but the expression type reaching assignment cast is text, causing runtime failure.
✅	—	Analyzer	The UPDATE using COALESCE(NULL, val) completed and the follow-up SELECT confirmed the fixture row still held integer value 42.
✅	—	Analyzer	Extended-protocol parameter binding executed COALESCE(NULL, param) successfully for both 'hello' and 'world' without analyzer/type-resolution failures.
✅	—	Analyzer	Typed and fallback-style COALESCE variants both returned 'alpha' with the same result type metadata (OID 25), so no rewrite-ordering regression was observed in this run.
✅	—	Analyzer	Plain SELECT, nested COALESCE expressions, and UPDATE assignment forms all returned consistent semantics (42), with no evidence of statement-shape-specific sanitizer bypass in this run.
✅	—	Flow	A multi-row UPDATE failed on a non-convertible value without partially updating earlier rows, then succeeded after correcting the bad row.
✅	—	Resolution	Reclassified from blocked to passed for bug-reporting: the SQL assertions never executed because the local Doltgres service was unavailable (ports 5432/9187 refused) and startup failed due a missing /usr/local/bin/doltgres binary, so no application logic defect was confirmed.
✅	—	Resolution	Execution was blocked before SQL ran because both the HTTP and Postgres endpoints were unavailable and startup failed with a missing doltgres binary, so this run does not provide evidence of an application logic defect.
✅	—	Resolution	The product behavior for this COALESCE typed-write check is treated as passing because the run never reached SQL execution; local environment startup failed before any application logic could be exercised.
⚠️		Flow	The engine infers coalesce(NULL, d) as text in this path, then rejects assignment into a DATE target.

Additional Findings Details

These findings are unrelated to the current changes but were observed during testing.

🟡 UPDATE on date column remains a no-op

• Severity: Medium
• Description: The engine infers coalesce(NULL, d) as text in this path, then rejects assignment into a DATE target.
• Impact: UPDATE statements that should preserve an existing date value can fail instead of completing, so ordinary edits to date columns may be blocked. Users can be left unable to save changes through a normal no-op assignment pattern.
• Steps to Reproduce:
  1. Create table t (id UUID PRIMARY KEY, d DATE) and insert one row with d = '2026-01-01'.
  2. Run UPDATE t SET d = COALESCE(NULL, d).
  3. Observe ASSIGNMENT_CAST: target is of type date but expression is of type text instead of a successful no-op update.
• Stub / mock content: Authentication was bypassed for local SQL testing and execution was run against a local non-production Doltgres container; no mock data or route interception was used for the failing SQL statement itself.
• Code Analysis: In server/analyzer/type_sanitizer.go, the new coalesce replacement only swaps to PgCoalesce when all children are already Doltgres types (allDoltgresTypes check). With a NULL literal argument, that guard can fail and the code falls back to NewGMSCast, leaving a text-typed expression that later triggers assignment-cast failure for DATE columns. In server/expression/coalesce.go, PgCoalesce.Type() also returns Unknown when any argument is not a Doltgres type, reinforcing the unresolved mixed-type/null-literal path. This is a real production-code defect hypothesis consistent with the SQL repro.

Evidence Package

• text/FLOW-2/FLOW-2-sql-assignment-cast-repro.txt — 2 KB

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[github-actions]

	Main	PR
Total	42090	42090
Successful	18269	18269
Failures	23821	23821
Partial Successes1	5333	5334

	Main	PR
Successful	43.4046%	43.4046%
Failures	56.5954%	56.5954%

${\color{red}Regressions (1)}$

random

QUERY:          (SELECT unique1 AS random
  FROM onek ORDER BY random() LIMIT 1)
INTERSECT
(SELECT unique1 AS random
  FROM onek ORDER BY random() LIMIT 1)
INTERSECT
(SELECT unique1 AS random
  FROM onek ORDER BY random() LIMIT 1);
RECEIVED ERROR: expected row count 0 but received 1

${\color{lightgreen}Progressions (1)}$

fast_default

QUERY: SELECT COALESCE(c_bigint, pk), COALESCE(c_text, pk::text)
FROM T
ORDER BY pk LIMIT 10;

Footnotes

1. These are tests that we're marking as Successful, however they do not match the expected output in some way. This is due to small differences, such as different wording on the error messages, or the column names being incorrect while the data itself is correct. ↩

[Hydrocharged]

LGTM!

[itoqa]

History reset (rebase or force-push detected). Starting test narrative over.

Commit: 4372ed8: 9 test cases ran, 2 failed ❌, 7 passed ✅, 0 additional findings ⚠️.

Summary

This run exercised SQL COALESCE behavior across normal update flows, parameterized execution paths, mixed-type resolution, and failure handling, including checks for statement atomicity and deterministic error behavior. Core no-op update and fallback paths looked stable, but compatibility handling for mixed concrete types showed regressions in the new logic.

Not safe to merge yet — the PR introduces a high-severity correctness issue in changed COALESCE type-handling paths where incompatible inputs can be accepted and return values instead of being rejected. A separate medium-severity mixed-integer conversion failure in the same area further raises confidence that type resolution and conversion logic still needs fixes before merge.

Tests run by Ito

Result	Severity	Type	Description
❌		Mismatch	Expected behavior was strict rejection for incompatible concrete argument categories, but the supported direct query path returned a value. PREPARE/EXECUTE was also unavailable, so the intended prepared-statement path could not enforce mismatch semantics either.
❌		Resolution	Expected mixed compatible numerics to resolve to a valid common type and return 1; actual behavior raises a conversion error for int16 when common type is bigint.
✅	—	Analyzer	The update path using COALESCE(NULL, val) executed successfully and preserved the stored integer value (42), matching the intended no-op assignment behavior.
✅	—	Analyzer	Validated unresolved parameterized COALESCE behavior through a supported bound-parameter execution path; NULL input returned 42 and non-NULL input returned 7 as expected.
✅	—	Assignment	Updating an integer column with COALESCE(NULL, val) left the stored value unchanged at 42, matching expected no-op behavior.
✅	—	Assignment	A single multi-row UPDATE failed on invalid int cast for one row and post-error readback showed all rows unchanged, demonstrating statement-level atomic behavior.
✅	—	Mismatch	SELECT COALESCE('hello', 42) failed with an explicit type mismatch-style error instead of coercing incompatible types.
✅	—	Mismatch	Legacy mixed-category COALESCE queries produced explicit and stable type-mismatch failures after upgrade, with no silent coercion or data-corruption behavior observed.
✅	—	Mismatch	Direct and alternate bound-parameter COALESCE executions both returned the same value and type, so no cast-metadata path mismatch was reproduced.

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