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Record and repair embedding model drift - #77

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Summary

A vectorizer that inherits follows pgedge_vectorizer.model as it changes, so a chunk table can end up holding vectors from two models with nothing reporting it. Those rows are not merely stale: similarity between two models' vectors is noise, so they are effectively invisible to search. Where the widths match, and text-embedding-3-small to text-embedding-ada-002 is both plausible and 1536 either way, the worker's dimension check cannot see it either.

set_embedding_model() guards the per-vectorizer path and cannot guard this one, because the extension does not own that GUC and cannot intercept every way it changes. So rather than pretend to:

  • Every chunk records the provider and model that produced its vector, written by update_embedding() in the same statement as the embedding so the two cannot disagree. Provider as well as model, because a model name alone is not an identity.
  • embedding_model_status() reports, per vectorizer, how many embedded chunks came from what it would use now, how many from something else, how many predate the columns, and which pairs are actually present. Shaped so Add vectorizer_status for embedding coverage and backlog #73's vectorizer_status view can absorb its columns unchanged once that lands.
  • reembed() repairs it. The obvious remedy does not work: set_embedding_model(force_reembed => true) takes its no-op branch here, since an inheriting vectorizer's effective model already is the new one.

The judgement calls

Unknown rows are reported apart but repaired anyway. A row with nothing recorded predates the columns and may well be current, so the report counts it separately rather than calling it drift. reembed() has to act rather than describe, and the safe reading of a row that cannot be shown to be current is that it needs doing again, so it goes. On a freshly upgraded installation that is every row; the docs say so.

A width change takes everything. At a matching width only the not-current rows are cleared and requeued. If the effective model is a different width the column has to be altered, which needs every embedding cleared, so every chunk is requeued whether it had drifted or not. A notice says which happened.

No confirmation flag on reembed(). Unlike set_embedding_model(), whose re-embed is a surprising consequence of a settings change, this function does what its name says. It raises a notice with the count, because the cost lands on a metered provider.

Two things found on the way

  • The upgrade script has to alter existing chunk tables itself. enable_vectorization() adds the columns to a chunk table it finds without them, but nothing re-runs it on upgrade, and the worker writes both columns in the same statement as the embedding. An existing installation would have failed every embedding write until someone happened to re-enable a vectorizer. 012_upgrade_1_1_to_1_2.pl now compares an upgraded chunk table's columns against a freshly created one, which is the check that would have caught it.
  • update_embedding() did not quote the chunk table name, so it would have failed for a schema-qualified source, where the generated name is one identifier with a dot in it rather than a qualified reference. The same family as the bug CodeRabbit caught in Add vectorizer_status for embedding coverage and backlog #73. Fixed in passing, since it is the statement being changed.

Test plan

  • model_drift regression test: the columns exist; four chunks in four states (current, drifted, unknown, unembedded) and the counts the report makes of them; unembedded chunks excluded from every count; narrowing by table and column; reembed() at a matching width clearing only the not-current rows and leaving the current one embedded; a vectorizer that is entirely current queuing nothing; a width change clearing everything and altering the column to vector(768); token_count and sparse_embedding intact throughout; the error for an unregistered vectorizer.
  • 013_embedding_model_recorded.pl: a real worker against a fake provider, two vectorizers on different models, asserting each table's rows record its own model rather than the GUC, that no embedded chunk is left without one, that the report sees everything as current, and that reembed() then queues nothing.
  • Full suite green on PostgreSQL 18.4: 23 pg_regress tests, 90 TAP tests.
  • Upgrade path exercised by hand as well: 1.1 install with a vectorizer and chunks, upgraded, chunk table gains both columns.

Note on the base branch

Stacked on #74, which this needs for the registry's provider and model columns, so the diff carries that work and #72's until they land. It targets main because CI only runs on PRs based on main, master or develop.

Closes #75

The chunking code in C has always sized chunks with a four-characters-per-token
estimate that rounds up, whilst the three plpgsql paths that actually write the
token_count column open-coded the same estimate as length(chunk_text) / 4, which
truncates. The two therefore disagreed by a token on most chunks, and on
anything shorter than four characters the plpgsql paths stored a zero that the
BM25 scoring path in worker.c then had to clamp back up to one. Since
token_count feeds the BM25 document-length normalisation, by way of
AVG(token_count) in bm25.c and the per-chunk value in worker.c, hybrid search
scored chunks written by the trigger slightly differently from chunks written by
the C chunker.

Expose the existing C counter as pgedge_vectorizer.count_tokens(text) and call
it from enable_vectorization(), vectorization_trigger() and recreate_chunks(),
so that there is one definition of the rule rather than two. It is declared
STABLE rather than IMMUTABLE deliberately: the estimate is defined in terms of
pgedge_vectorizer.model, which does not matter whilst the counter ignores the
model, but would quietly invalidate an expression index or a cached plan the
moment it stops doing so.

Existing chunk tables are left alone. The stored values are an approximation
either way, and rewriting every chunk table to correct a single token is not a
trade worth making on upgrade.

This is the first change for 1.2, so the extension version moves on and
sql/pgedge_vectorizer--1.1--1.2.sql carries the upgrade; the 1.1 scripts are
untouched.
The four provider implementations read pgedge_vectorizer.model straight from
the GUC, and the provider interface had nowhere to put a model, so nothing
could ask for an embedding from anything other than whatever the database was
globally configured to use. That is the obstacle to a per-vectorizer model, and
it is removed here rather than worked around by setting the GUC around each
request: a provider that silently depends on ambient global state cannot be
asked to embed with anything else, and mutating that state inside the worker's
batch loop would have made the error paths considerably harder to reason about.

generate() and generate_batch() therefore take the model explicitly, and the
providers use the argument. Every existing call site passes the GUC, so
behaviour is unchanged.

generate_embedding() and detect_embedding_dimension() gain optional provider
and model arguments on the same rule that will apply to the registry: NULL
means fall back to the GUC. Neither is STRICT any more, since a STRICT function
would return NULL before that argument could reach the C. The dimension probe
needs this in particular, because a vectorizer created with an override needs
the dimension of the model it names, not of whatever the GUCs happen to say.
pgedge_vectorizer.vectorizers gains nullable provider and model columns, and
enable_vectorization() gains matching parameters in ninth and tenth position so
that existing positional calls are untouched. NULL means inherit the GUC at the
time the work runs rather than a copy taken at creation, so an installation
that never sets either behaves exactly as it did.

Where the dimension is not given, the probe now asks about the model this
vectorizer will actually use rather than the one the GUCs name, which would
otherwise size the vector column against the wrong model whenever an override
was passed.

Nothing reads the columns yet; the worker does that in the next commit.
The model was one GUC applied to every table in the database, which is the
wrong granularity: a table of short product titles and a table of long
technical documents are rarely well served by the same model, and there was no
way to embed one table locally through Ollama whilst another went to a hosted
provider.

A vectorizer now records its own provider and model, both nullable, NULL
meaning inherit. Inheritance resolves when the work runs rather than being
copied at creation, so an installation that sets neither carries on exactly as
before. They can be pinned at enable_vectorization() or changed afterwards with
the new set_embedding_model().

The worker resolves inheritance in the query that fetches a batch, with a left
join against the registry and COALESCE against the GUCs, so the rule lives in
one place and an item whose vectorizer has since been disabled falls back
through the same expression rather than needing a special case. A batch is
selected by age across every vectorizer at once, so it can hold items for
several models, and a request carries one; the batch is therefore grouped by
(provider, model) before the existing loop runs and batch_extent() breaks on
the same key. Sorting rather than merely breaking the run matters, because two
tables' items alternating in time would otherwise give requests of one item
each. The provider is resolved per request instead of once per batch.

set_embedding_model() refuses to change a vectorizer that already has
embeddings unless force_reembed is passed. The refusal keys on the model
changing rather than the dimension changing, which is the case worth guarding:
a dimension change is caught before any write by the existing check in the
worker, whilst a change between two models of the same width, say
text-embedding-3-small and text-embedding-ada-002 at 1536 each, would leave the
old vectors in place, correctly shaped and meaningless beside the new ones,
with nothing reporting a problem. With force_reembed the embeddings are cleared,
the column rewidened if needed, the queue cleared and every chunk requeued, all
in one transaction. Chunk rows, token counts, sparse embeddings and the BM25
statistics are untouched, since none of them depends on the embedding model.

Three things fell out along the way.

generate_embedding() and detect_embedding_dimension() take an optional provider
and model, and can no longer be STRICT, because NULL has to reach the function
to mean "use the GUC". That makes generate_embedding(NULL) raise rather than
return NULL, which is what the function always meant to do and said so in its
own code, unreachably.

The array of chunk tables that disable_vectorization() drops was collected with
no ORDER BY, so the notices came out in whatever order the scan returned, and
adding registry columns changed it. It is ordered now.

Adding defaulted parameters to enable_vectorization() with CREATE OR REPLACE
defined a second function rather than replacing the old one, leaving two
overloads on any upgraded installation, an eight-argument call reaching a body
that knew nothing of the new columns, and COMMENT ON FUNCTION failing as
ambiguous, whilst a fresh install was perfect throughout. pg_regress installs
whatever default_version says, so it can never see this; 012_upgrade_1_1_to_1_2
builds a 1.1 installation, upgrades it, and compares its functions, columns and
views against a fresh 1.2, which is the shape of check that catches the whole
class rather than this one instance.

Closes #27
test/expected/ carries numbered variants of embedding.out for the cases where
provider API keys are actually available, and pg_regress passes if the output
matches any of them. Only the default one was updated for the new
generate_embedding() signature and for NULL input now raising, so every
platform without keys was happy and the macOS runner, which has them, failed
against embedding_4.out.

The two changed lines are common to all five, so all five now carry them.
Widening where the model comes from, from one GUC to a registry column and a
SQL argument, broke assumptions in several places that were safe whilst it was
a single trusted setting.

The model was interpolated raw into every provider's request body, and into
Gemini's URL path. Escaped now in all four, once in the shared OpenAI-format
builder that OpenAI and Voyage use and once each in Ollama and Gemini, with a
percent-encoding helper for the URL segment, where a '/' or '?' would have sent
the request to a different endpoint.

A rate limit deferred every remaining item in the pull and charged each one a
deferral, which was right whilst a pull could only carry one provider's work.
Now that a batch can span providers, that spent deferrals belonging to a
provider that had not refused anything and, once they ran out, failed its items
outright. Only the refused provider's items are deferred; the rest go back to
pending uncharged for the next pull.

set_embedding_model() computed the new dimension and altered the column only
when the vectorizer had chunks, so an empty one kept the width it was created
with and failed every embedding written after the change, which is precisely
the failure the function exists to prevent. The dimension is resolved and the
column altered whichever it is, whilst clearing embeddings and requeuing stay
conditional. That does mean a call without an explicit dimension always probes
the provider, as enable_vectorization() does, which the reference now states.

The requeue omitted max_attempts, taking the column default of 3 rather than
pgedge_vectorizer.max_retries as every other queue insert does.

The worker's inheritance expression handled NULL but not an empty string,
which resolve_provider() and resolve_model() in embed.c both treat as inherit;
NULLIF puts them on the same footing.

Two documentation corrections. The data-management guidance still told users to
drop the chunk table and enable vectorization again after a dimension change,
which throws away chunks, sparse embeddings and BM25 statistics that were never
wrong. And the configuration example passed NULL as the model without saying
that the provider defaults to NULL too, so it reverts alongside unless named
again.

Raised by CodeRabbit on #74.
The call restoring ptm_named's model left provider at NULL, so the reset case
that follows started from a vectorizer with nothing pinned and could not show
that the provider clears alongside the model. Pinning both first makes the
assertion mean something. The provider named matches the GUC, so the effective
values still do not move and the case remains the no-op it is there to cover.

Raised by CodeRabbit on #74.
A vectorizer whose provider and model are NULL inherits the GUCs, and
inheritance resolves when the work runs rather than being copied at creation.
Changing pgedge_vectorizer.model therefore re-points every inheriting
vectorizer at once, leaving a chunk table holding vectors from the old model
beside new ones from the new. Similarity between two models' vectors is noise,
so those rows become effectively invisible to search rather than merely stale,
and where the widths match, as they do between text-embedding-3-small and
text-embedding-ada-002, the dimension check in the worker cannot see it either.

set_embedding_model() guards the per-vectorizer path and cannot guard this one:
the extension does not own that setting and cannot intercept every way it
changes. Rather than pretending otherwise, each chunk now records the provider
and model that produced its vector, written by update_embedding() in the same
statement as the embedding so the two cannot disagree, and
embedding_model_status() reports where that differs from what the vectorizer
would use now. That diagnoses instead of preventing, but it catches drift
whatever its cause, including a setting changed months ago.

Reporting alone would leave a number and nothing to do about it, and the
obvious remedy does not work: set_embedding_model(force_reembed => true) takes
its no-op branch here, because an inheriting vectorizer's effective model
already is the new one. reembed() therefore clears and requeues every chunk not
known to have come from the effective provider and model. Rows with nothing
recorded are counted apart in the report, since they predate the columns and
may well be current, but reembed() treats them as needing redoing, a row that
cannot be shown to be current being one to do again; on a freshly upgraded
installation that is every row, which the documentation says plainly. A change
of embedding width takes every chunk with it, drifted or not, because a column
cannot hold two widths.

Two things found on the way.

The columns had to be added to chunk tables by the upgrade script itself.
enable_vectorization() adds them to a chunk table it finds without them, but
nothing re-runs it on upgrade, and the worker writes both columns in the same
statement as the embedding, so an existing installation would have failed every
embedding write until someone happened to re-enable a vectorizer. 012 now
compares an upgraded chunk table's columns against a freshly created one, which
is the check that would have caught it.

update_embedding() interpolated the chunk table's name into its UPDATE without
quoting, so it would have failed for a schema-qualified source, where the
generated name is one identifier with a dot in it rather than a qualified
reference. Fixed in passing, since it is the statement being changed.

Closes #75
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Up to standards ✅

🟢 Issues 2 high · 5 medium

Results:
7 new issues

Category Results
Compatibility 2 high (1 false positive)
Complexity 5 medium

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📝 Walkthrough

Walkthrough

The extension is upgraded to version 1.2. It adds per-vectorizer provider and model overrides, embedding provenance, model-drift status reporting, guarded model changes, and re-embedding functions. Provider callbacks and worker batching now use explicit provider/model pairs. Token counting is exposed through SQL and shared by chunking paths. Cleanup, sparse embedding, BM25, queue handling, and hybrid search are updated. Documentation and regression tests cover upgrades, model changes, provider routing, token counts, and provenance.

Fixed issue severity: Medium

Priority: ➖ Normal

Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to b055d

This release improves embedding-model provenance and repair, but invalid provider configuration can still stall embedding queue processing and some generated chunk table names can fail dimension checks. These issues should be resolved before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name Status Explanation Resolution
Out of Scope Changes check ⚠️ Warning The PR includes the required #75 changes, but it also carries broader #74 and #72 work, including per-vectorizer model configuration, hybrid-search changes, queue behavior changes, and the complete 1.… Rebase onto the completed prerequisite work or split the changes into separate pull requests. Keep this PR limited to #75-related provenance recording, drift reporting, reembedding, upgrade support, and directly required tests and fixes. Do…
✅ Passed checks (4 passed)
Check name Status Explanation
Title check ✅ Passed The title clearly summarizes the primary change: recording and repairing embedding model drift.
Description check ✅ Passed The description directly explains embedding model drift, provenance tracking, status reporting, re-embedding, upgrade handling, and tests.
Linked Issues check ✅ Passed The PR satisfies issue #75 by recording provider and model provenance, reporting current, drifted, unknown, and unembedded states, and repairing affected embeddings with reembed(). It also handles dim…
Docstring Coverage ✅ Passed Docstring coverage is 91.30% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 23 functions across 10 files. (2 skipped: 2…
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Explanation

The PR includes the required #75 changes, but it also carries broader #74 and #72 work, including per-vectorizer model configuration, hybrid-search changes, queue behavior changes, and the complete 1.2 implementation.

Resolution

Rebase onto the completed prerequisite work or split the changes into separate pull requests. Keep this PR limited to #75-related provenance recording, drift reporting, reembedding, upgrade support, and directly required tests and fixes. Document any prerequisite dependency that must remain in this PR for compilation or integration reasons.

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Actionable comments posted: 6

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⚠️ Outside diff range comments (1)
src/worker.c (1)

1879-1884: 🩺 Stability & Availability | 🟡 Minor | ⚡ Quick win

Quote the chunk table name in the dimension probe, as update_embedding() now does.

chunk_tables[idx0] is interpolated raw into '%s'::regclass. Per the comment added at Lines 2250-2253, the generated chunk table name for a schema-qualified source is a single identifier that contains a dot. regclass parses that text as schema.table, so the lookup fails and SPI_execute() raises, which aborts the batch before any item is charged. Line 2259 already repairs the same defect in update_embedding().

🐛 Proposed fix
 					ret_dim = SPI_execute(psprintf(
 						"SELECT atttypmod FROM pg_attribute "
-						"WHERE attrelid = '%s'::regclass "
+						"WHERE attrelid = %s::regclass "
 						"AND attname = 'embedding'",
-						chunk_tables[idx0]),
+						quote_literal_cstr(quote_identifier(chunk_tables[idx0]))),
 						true, 1);
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In `@src/worker.c` around lines 1879 - 1884, Update the dimension probe’s SQL in
the SPI_execute call to quote chunk_tables[idx0] as a single identifier,
matching the handling in update_embedding(), so schema-qualified generated names
containing dots resolve correctly through regclass.
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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@sql/pgedge_vectorizer--1.2.sql`:
- Around line 727-734: Normalize empty provider and model values with NULLIF
before COALESCE in both old and new effective-setting calculations in both SQL
definitions, including the corresponding set_embedding_model implementations.
Ensure the normalized effective provider and model values are used for
comparisons and passed to detect_embedding_dimension(), preserving GUC
inheritance for empty strings.
- Line 1827: Update both hybrid_search() definitions to resolve provider and
model alongside chunk_table in each lookup branch, preserving NULL values for
GUC inheritance, then pass these values to generate_embedding(p_query,
v_provider, v_model). Apply the same change in the 1.1-to-1.2 migration so it
redefines hybrid_search() consistently.

In `@src/embed.c`:
- Around line 50-55: Update resolve_model to validate pgedge_vectorizer.model
before returning it, rejecting both NULL and empty values with a local
configuration error; otherwise return the configured model or existing default
so provider_build_openai_request never receives an invalid model.

In `@src/worker.c`:
- Around line 1657-1658: Add a uniqueness constraint or unique index for
vectorizers.chunk_table, after resolving any existing duplicate values, so the
worker’s join in the queue-processing query cannot produce duplicate rows or
repeated embedding and bm25_update_idf_stats() calls.
- Around line 1846-1854: Update process_queue_batch so failures from
get_embedding_provider or provider->init use the existing per-request failure
path instead of raising immediately. Record failure for every item in the
request’s batch_count, mark it non-rate-limited, clear or update
failed_item_queue_id consistently, and continue processing the next request so
unrelated providers remain pending.
- Around line 2120-2126: Update process_queue_batch() and the file-static
provider_cooldown_until state to maintain separate cooldown deadlines per
resolved provider instead of one global deadline. When filtering or selecting
queue items, apply only that item’s provider cooldown; preserve unblocked
providers’ access and ensure a 429 updates only the provider that received it.

---

Outside diff comments:
In `@src/worker.c`:
- Around line 1879-1884: Update the dimension probe’s SQL in the SPI_execute
call to quote chunk_tables[idx0] as a single identifier, matching the handling
in update_embedding(), so schema-qualified generated names containing dots
resolve correctly through regclass.

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Comment thread src/worker.c
hybrid_search() embedded the query with the GUCs, which was right whilst that
was the only place a model could come from. Since a vectorizer can pin its own,
a query embedded by one model was being compared against chunks embedded by
another: meaningless distances where the widths match, and an outright error
where they do not, which is the failure this whole line of work exists to
prevent, arriving through the search path instead. It now looks the provider
and model up alongside the chunk table and passes them through, NULL and all,
so a vectorizer that has pinned nothing behaves as before. The 1.1 script never
redefined the function, so the upgrade carries a full replacement.

The worker's join to the registry could match a queue row twice, because only
(source_table, source_column) is unique and two vectorizers can be pointed at
one chunk table with an explicit chunk_table_name. The item would then be
embedded twice and counted twice into the BM25 corpus statistics. A LATERAL
with LIMIT 1 makes the join return one row whatever the registry holds; a
unique constraint on chunk_table would be the stronger fix but needs a story
for installations that already have duplicates, which is separate work.

set_embedding_model() compared raw stored values where the worker,
embedding_model_status() and reembed() all treat an empty string as inherit,
so an empty override would have been seen as a change by one of the four and
not by the other three, and would have reached the dimension probe as a model
name of ''. NULLIF everywhere, and the probe now asks about the effective
values.

resolve_model() returned an unset model GUC unvalidated, where resolve_provider()
already refuses one. The providers interpolate it straight into their request
bodies, so an empty setting put "model":"" on the wire and a NULL one would have
been dereferenced whilst escaping it.

One finding declined. CodeRabbit proposed charging a provider that cannot be
resolved to the items of its request, so that the rest of the pull proceeds.
The observation behind it is right, and is now issue #78: since a vectorizer can
name its own provider, one bad name stops every other vectorizer in the
database. The prescription is not, because 005_batch_failure_backoff.pl exists
to prevent exactly that and injects exactly this fault: charging a blameless
item for a misconfigured provider works through the queue retiring one innocent
row per max_attempts cycles, so a single mistyped provider name would mark the
whole queue failed. Fixing it properly means skipping the group without
charging it whilst still reaching the batch backoff, which needs a way to report
a batch-level fault without an exception. That is more than this change should
carry.

Raised by CodeRabbit on #77.
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Changing pgedge_vectorizer.model silently re-points every inheriting vectorizer

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