[alxmrs]

Why

A training step of xarray-sql's GPT on DuckDB spends part of its time on work ddxdb repeats every call (#216):

• Catalog reads. sql_all read the caller's catalog for every plan, key and view it asked for.
• Copies the planner never reads. Every call DESCRIBEd each step's table and copied it into the planner, then dropped the copy. Only a step bound while it is planned reads a copy: none in a replayed call, and none of a table no later step reads.
• Repeated SQL in prepare. A prepared call wrote each step's SQL again (to_sql).

How

• One catalog read per call. DuckDBBackend._one_catalog holds the catalog, the planner's mirror of it and the current schema for one call's rewrite. That rewrite writes only ddx's own __ddx_ tables, which the catalog leaves out. The caller's rewritten statements run outside it, since they may change the catalog.
• The planner copies a table only to plan a read of it. materialize marks the table it writes as needing a fresh copy. select_all copies the table it is asked for, and plan_sql those the SQL names, just before planning. Each of ddx's tables has a name no other object has, so one the SQL doesn't name, it doesn't read. A replayed call plans nothing, so it copies nothing. Neither does a table no later step reads, such as the value and the gradients. drop_table drops only a copy that exists.
• compile keeps each plan's SQL, so a prepared call skips to_sql.

Not done: DuckDB prepared statements (the issue's ask 1), or keeping step tables between calls (ask 3, which pays only with ask 1). Probed on DuckDB 1.5.5:

• Stale statistics. A prepared statement keeps the statistics it was optimized with. After DELETE and INSERT, a prepared count(*) … WHERE x > 5 returned 0 rows instead of 3.
• Rebinding. DuckDB rebinds every prepared statement once a temporary object is made or registered, and xarray-sql's training loop registers its batch and weights each step. So the plans would be planned again anyway.

DuckDB's own planning is about 140 ms of the step, by its profiler: 60 ms binding, 73 ms in its optimizers. The EXPLAINs in the issue's estimate cost more than planning alone.

Tests

test_duckdb.py:

• test_a_later_prepared_call_asks_the_planner_for_nothing: after new weights are registered, a later call is right and sends the planner no statement. On main it sends 8.
• test_the_planner_copies_only_the_tables_it_plans_a_read_of: over two statements, sql_all copies none of the values or gradients, which no later step or statement reads.
• test_an_sql_call_reads_the_connection_s_catalog_once: two statements over a view read duckdb_columns() once. On main they read it 4 times.

xarray-sql's GPT step on DuckDB, batch 4, with the same Rust. The counts are exact. The times are two interleaved runs each, median of five calls per run, on a machine at load 5–8:

	Statements to the connection	To the planner	Time outside the steps' CREATE TABLE AS
sql_all, main	376	238	399, 457 ms
sql_all, this PR	234	194	175, 202 ms
prepare, main	219	136	163, 153 ms
prepare, this PR	151	0	64, 61 ms

prepare also skips to_sql, about 10 ms a call.

Next

The demo calls sql_all. Calling ddxdb.duckdb.prepare instead skips the catalog, keys and views on later calls: about 125 ms less per step on main, before this PR.

Closes #216.

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Summary by CodeRabbit

• Bug Fixes
  • Improved consistency when running prepared queries and processing multiple statements that reference views, reducing unnecessary catalog lookups during a single operation.
  • Fixed cases where temporary tables could be left behind or planner state could become outdated between runs.
  • Improved handling of tables created or removed during query planning, helping subsequent operations reflect the current database state.
  • Prepared runs now avoid repeating planner queries and temporary table copies when they are not needed.

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• python/ddxdb/ddxdb/duckdb.py
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Walkthrough

DuckDBBackend now caches catalog-dependent reads during scoped rewrites, manages planner copies of ddx tables, and provides compiled SQL methods. Tests cover prepared-call replay, planner-copy behavior, cleanup, and catalog reads across statements.

Changes: DuckDB backend

Layer / File(s)	Summary
Scoped catalog and location caching python/ddxdb/ddxdb/duckdb.py	Scoped calls cache catalog and database/schema lookups. Synchronization, view lookup, and session keys use the cached location.
Compiled execution and planner-table lifecycle python/ddxdb/ddxdb/duckdb.py	Compiled SQL is used for materialization and row checks. Planner copies are created or refreshed when stale and dropped only when tracked as copied.
Scoped rewrites and validation python/ddxdb/ddxdb/duckdb.py, python/ddxdb/tests/test_duckdb.py	sql_all and prepared runs scope catalog caching around rewriting. Tests check prepared replay, planner-copy tracking and cleanup, and catalog reads across statements.

Priority: Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Refactor · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant PreparedRun
  participant DuckDBBackend
  participant PlannerConnection
  participant CallerConnection
  PreparedRun->>DuckDBBackend: rewrite within catalog-cache scope
  DuckDBBackend->>PlannerConnection: plan using current or refreshed table copies
  DuckDBBackend->>CallerConnection: execute rewritten statements after scope

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Possibly related issues

• xqlsystems/ddx#216: The changes address repeated catalog reads and planner-copy work. They do not implement prepared statements or persistent step tables requested by the issue.

Possibly related PRs

• xqlsystems/ddx#214: Defines compiled-plan hooks that this PR implements in DuckDBBackend.
• xqlsystems/ddx#203: Adds the prepared-run and session-key paths that this PR changes.
• xqlsystems/ddx#210: Adds planner-copy tracking that this PR changes to use lazy copies and stale-state refresh.

Merge Risk: Minimal · up to 11661

The reviewed changes preserve catalog isolation, prepared execution, and temporary-table cleanup; no material current-head risk remains.

Security architecture risk: Low · up to 3afc3

Execution retains the caller connection’s existing authority. Catalog reuse ends before caller statements execute, and prepared replay retains schema-sensitive invalidation. Remaining uncertainty concerns recovery after temporary-object cleanup failures; no material security regression was established.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The effective SQL execution scope remains whatever the caller connection can access, not merely generated temporary tables. An untrusted SQL source would inherit that existing authority; the inspected changes do not establish additional cross-connection or cross-tenant access.

Trust Boundaries and Controls

• observed — Replay identity includes catalog schemas and view definitions, current database/schema, and relevant settings. Replay checks that identity before materializing jobs. Planning also retains rejection of ambiguous same-name catalog objects rather than silently binding a different object.

Resilience and Maintainability Implications

• inferred — Cleanup attempts all owned result tables and preserves the original execution error. A failed DROP can nevertheless leave an empty planner copy after tracking is cleared. Unlike the baseline’s unconditional planner DROP, a later direct retry can now skip that copy. Runtime reachability and material security impact of this failure path remain unverified.

Hardening Proposals

• proposed — If recovery after failed temporary-object deletion is required, retain failed planner-copy cleanup provenance or reconcile leftover copies before backend reuse. This is a failure-recovery improvement, not an established data-exposure finding.

Pre-merge checks: 3 · 2

Failed checks: 2 warnings

Check name	Status	Explanation	Resolution
Linked Issues check		Active issue [#216] has three coding requirements. The PR improves per-call behavior, but it does not meet the issue requirements. The diff retains SQL with compile, then executes `CREATE OR REPLACE…	Implement the remaining [#216] requirements, or update the issue and PR scope before claiming issue compliance. Add DuckDB prepared-statement execution with plan-reuse tests. Add cross-call catalog-change detection and tests that skip `_syn…
Docstring Coverage		Docstring coverage is 44.44% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 3 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main performance change: avoiding repeated planning on later DuckDB calls.
Out of Scope Changes check		The changed DuckDB backend and test_duckdb.py tests support the PR's narrower performance objective and relate to [#216]. The per-call catalog scope, lazy planner copies, retained SQL, and related t…

Full details: Out of Scope Changes check

Explanation

The changed DuckDB backend and test_duckdb.py tests support the PR's narrower performance objective and relate to [#216]. The per-call catalog scope, lazy planner copies, retained SQL, and related tests are connected to reducing repeated DuckDB work. No unrelated change is established.

Full details: Linked Issues check

Explanation

Active issue [#216] has three coding requirements. The PR improves per-call behavior, but it does not meet the issue requirements. The diff retains SQL with compile, then executes CREATE OR REPLACE TEMP TABLE ... AS; it does not use DuckDB prepared statements. _one_catalog caches catalog data only for one rewrite or prepared run, not across unchanged calls, so the issue's cross-call skip of _sync, plan_key, and view_sql is not established. drop_table still removes ddx step tables after a call, so the issue's retained-table requirement is not implemented. The added tests cover per-call catalog reads, lazy planner copies, and replay with no new planning. They do not test the three missing requirements.

Resolution

Implement the remaining [#216] requirements, or update the issue and PR scope before claiming issue compliance. Add DuckDB prepared-statement execution with plan-reuse tests. Add cross-call catalog-change detection and tests that skip _sync, plan_key, and view_sql when the catalog is unchanged. Retain step tables between calls and test the required update behavior.

Full details: Docstring Coverage

Explanation

Docstring coverage is 44.44% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 9 functions across 2 files.

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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Inline comments:
Review comments at @python/ddxdb/ddxdb/duckdb.py:
- Line 311: Update _plan so _copy receives only stale tables read by the SQL
being planned; determine those reads before planning or recover when planning
identifies a missing copy. Ensure read tables have planner copies while
unrelated stale tables are not copied.

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[alxmrs]

LGTM