# SQL Poll Driver

Getting-started guide for the `Sql` driver — a **read-only** Equipment-kind driver that polls rows
out of a relational database and publishes them as OPC UA variables. For the design rationale read
[`docs/plans/2026-07-15-sql-poll-driver-design.md`](../plans/2026-07-15-sql-poll-driver-design.md);
for the build-vs-plan record read
[`docs/plans/2026-07-24-sql-poll-driver.md`](../plans/2026-07-24-sql-poll-driver.md).

## What it talks to

An ordinary SQL database — typically a MES / LIMS / historian staging table that some other system
already writes to. There is no protocol here and nothing to "connect" to a machine: the driver runs
`SELECT`s on a timer and turns cells into tag values.

**Three providers construct**: `SqlServer` (`Microsoft.Data.SqlClient`), `Postgres`
(`Npgsql`) and `Oracle` (ODP.NET managed — **Oracle 12.2 or later**, the floor the
`FETCH FIRST` paging needs). `SqlProvider` also declares `MySql` and `Odbc`, but those remain
**reserved names, not working backends** — authoring one fails the driver rather than silently
falling back.

Two provider-specific things are worth knowing before you author against them:

- **Identifier case folds, and the catalog gate absorbs it.** Postgres lower-cases and Oracle
  upper-cases unquoted identifiers. Every authored identifier is resolved to the catalog's own
  spelling, so an authored `widgets` binds to Oracle's `WIDGETS` rather than failing.
- **Oracle `NUMBER` infers as `Float64`.** Column metadata does not carry precision/scale to the
  type map, so an integer-shaped `NUMBER` still infers double. Use the per-tag `type` override
  (see [Optional per-tag fields](#optional-per-tag-fields)) when the OPC UA type matters.

**Read-only, structurally.** `SqlDriver` does not implement `IWritable`, so no configuration can
produce a write. The `allowWrites` config field exists and is **inert**; the factory *warns* when it
is authored `true` rather than failing, on the grounds that the flag cannot do harm but an operator
who believes writes are enabled can.

## Tag models

A tag says how to find its value. Two models ship; a third is deferred.

### `KeyValue` — the EAV shape

One row per tag. The driver matches `keyColumn = keyValue` and reads `valueColumn`.

```json
{
  "model": "KeyValue",
  "table": "dbo.TagValues",
  "keyColumn": "TagName",
  "keyValue": "Line1.Oven.Temp",
  "valueColumn": "Value",
  "timestampColumn": "UpdatedUtc"
}
```

### `WideRow` — one row, many signals

The tag names its `columnName`; the row is picked either by a `whereColumn`/`whereValue` pair or by
newest-timestamp.

⚠️ **The selector keys live inside a nested `rowSelector` object**, not at the top level. A blob that
spells `whereColumn` / `whereValue` / `topByTimestamp` beside `columnName` carries *no* row selector
as far as `SqlEquipmentTagParser` is concerned: the tag is refused, the driver skips it, and it reads
`BadNodeIdUnknown` forever. The typed editor, the browse-commit mapper and the deploy gate all use
the nested form.

```json
{
  "model": "WideRow",
  "table": "dbo.LineStatus",
  "columnName": "OvenTemp",
  "rowSelector": { "whereColumn": "LineId", "whereValue": "L1" }
}
```

```json
{
  "model": "WideRow",
  "table": "dbo.Readings",
  "columnName": "OvenTemp",
  "rowSelector": { "topByTimestamp": "CapturedUtc" }
}
```

⚠️ **`topByTimestamp` needs an ordering column that is unique per row.** It is executed as an
`ORDER BY <column> DESC` with the dialect's single-row limit, so when the column ties the server
returns an arbitrary row among the tied ones — and the driver cannot warn about it, because the other
rows were discarded before the result set reached it (the `whereColumn`/`whereValue` form *does*
warn, on `Rows.Count > 1`). A `date`-typed column ties for every row written on the same day, and
browse-commit will nominate one. Prefer a `datetime2`/sequence column that is unique per row, or
point the tag at a latest-row view.

Both value-bearing fields — `whereValue` here and `keyValue` on the `KeyValue` model — may be
authored as a JSON string, number or boolean, and are captured as text. They are **bound as
parameters** either way, and matched back as text (the key cell is stringified), so a key column
that holds an INTEGER station id can be authored either `"7"` or `7`. A JSON object, array or `null`
is not a value and is refused. On **PostgreSQL** a string bind is sent untyped (`NpgsqlDbType.Unknown`,
`PostgresDialect.ConfigureParameter`, STAB-24) so the server infers its type from the compared column;
bound as `text`, a `"7"` against an `integer` key fails with `operator does not exist: integer = text`.

⚠️ **Known limitation — key normalisation.** Row slice-back matches the returned key cell by its *text* form
(`Convert.ToString`, ordinal), while the server now matches by the column's inferred type. An authored key the
server treats as equal but whose text differs therefore selects a row that the driver then cannot find, and the
tag reads **no-row**: `"007"` against an `integer` column (returns `7`), `"1.50"` against `numeric` (returns
`1.5`), and an upper-case GUID against a `uuid` column (returned lower-case). Author keys in the column's
canonical text form. SQL Server behaves the same way.
(They were read asymmetrically until 2026-08-10, so a numerically
authored `keyValue` refused the tag while the identically-bound `whereValue` was served — and the
deploy gate reported the key as *missing* when it was right there in the blob.)

### `Query` — deferred to P3

Named arbitrary-`SELECT`. **The v1 parser rejects it**, and that refusal is enforced end-to-end —
parser, validator, planner and the AdminUI editor all decline it rather than accepting the value and
ignoring it later.

## Optional per-tag fields

| Field | Effect |
|---|---|
| `timestampColumn` | Source timestamp for the value. Absent ⇒ the driver stamps the poll time. |
| `type` | Explicit OPC UA type override. Absent ⇒ inferred from the result set's column metadata. |

There is **no per-tag poll-interval field**. The interval is the subscriber's, floored by
`PollGroupEngine.DefaultMinInterval`, falling back to the instance's `defaultPollInterval` when the
subscriber names none. (A `pollIntervalMs` per-tag override was documented here for a while and never
existed in the parser — nothing reads it.)

## Driver config

```json
{
  "provider": "SqlServer",
  "connectionStringRef": "MesStaging",
  "defaultPollInterval": "00:00:05",
  "operationTimeout": "00:00:15",
  "commandTimeout": "00:00:10",
  "maxConcurrentGroups": 4,
  "nullIsBad": false
}
```

| Key | Default | Notes |
|---|---|---|
| `provider` | `SqlServer` | `SqlServer`, `Postgres` or `Oracle`. `MySql` / `Odbc` are reserved and rejected. |
| `connectionStringRef` | — | **A name, never a connection string.** See below. |
| `defaultPollInterval` | `00:00:05` | Applied to any group without its own interval. |
| `operationTimeout` | `00:00:15` | Client-side wall-clock deadline; a breach surfaces `BadTimeout`. |
| `commandTimeout` | `00:00:10` | ADO.NET server-side backstop. **Must be less than `operationTimeout`** — it is the backstop, not the deadline. |
| `maxConcurrentGroups` | `4` | Connection-pool guard: cap on group queries in flight. |
| `nullIsBad` | `false` | `false` ⇒ a NULL cell publishes **Uncertain**; `true` ⇒ **Bad**. |
| `allowWrites` | — | **Inert.** v1 has no write path; authoring `true` logs a warning. |

### Credentials never enter the deployment artifact

`connectionStringRef` is a **name** resolved at Initialize from the environment / secret store — e.g.
`Sql__ConnectionStrings__MesStaging`. There is no field that holds a connection string, so a sealed
deployment artifact cannot carry database credentials even by accident.

A useful side effect of the in-place reinit added for [#516](#): a **rotated credential is picked up
on the next config apply without a process restart**, because the driver re-resolves the reference
when it re-parses.

## How polling works

Tags are **grouped by source** and one query is issued per group, not per tag — `SqlGroupPlanner`
emits a `SqlQueryPlan` per distinct group key. Grouping is deterministic (groups appear in the input
order of their first member, members keep their input order, parameters keep first-appearance order),
so an unchanged configuration produces byte-identical SQL.

**Every key-value and where-pair query is row-capped (R06:S-1).** A key-value plan reads at most twice as
many rows as it has distinct keys and asks for one more (`TOP (2n+1)` / `LIMIT 2n+1` / `FETCH FIRST 2n+1 ROWS
ONLY`); a where-pair wide-row plan asks for two. The reader stops there even if a provider ignores the limit.

- **A duplicated key is refused per key.** When a key comes back more than once, only the tags on THAT key
  publish `BadConfigurationError`; every other key in the group reads normally, whatever else is present or
  absent. (Before this, a duplicate was last-row-wins Good.)
- **A gross excess fails the group.** Receiving the extra row past the cap means a history/events table or a
  non-unique where column, which cannot be attributed because the rest is unread, so every member of the group
  publishes `BadConfigurationError`. For a where-pair any second row is this case.
- Each case logs a per-plan rate-limited warning naming the table and the selector (or an example key).
  `topByTimestamp` keeps its single-row limit.
- **Upgrade note:** a deployment whose key-value source carries stray duplicate rows, or whose where column is
  not unique, will see those tags go Bad where they used to read an arbitrary row at Good.

Every such refusal, like the other refusals (absent row, NULL cell, uncoercible cell), is the database
answering: it advances `LastSuccessfulRead` and restores `Healthy` after a comms fault.

⚠️ **Group keys are case-sensitive on identifiers.** Two tags authored `dbo.T` and `DBO.T` plan as
**two** groups — one extra round-trip, not a correctness problem, but worth knowing if a group count
looks higher than expected.

## SQL injection: how the driver is safe

The two classes of authored string are handled differently, and deliberately so:

- **Value-bearing** fields (`keyValue`, `whereValue`) are bound as `DbParameter`s. They are never
  concatenated into command text.
- **Identifier-bearing** fields (`table`, `keyColumn`, `valueColumn`, `timestampColumn`,
  `columnName`, `whereColumn`, `topByTimestamp`) cannot be parameterised in SQL. They are validated
  against the live catalog — **`INFORMATION_SCHEMA`** on SQL Server and Postgres, the **`ALL_*`**
  views (`ALL_USERS` / `ALL_TABLES` ∪ `ALL_VIEWS` / `ALL_TAB_COLUMNS`) on Oracle — and
  dialect-quoted before they reach a command.

The parser deliberately does **not** sanitise either kind — sanitising a value would corrupt
legitimate data, and sanitising an identifier would mask an authoring error the catalog check reports
properly.

⚠️ **An unreadable catalog FAULTS the driver rather than rejecting every tag** (Gitea #496). This is
the safer failure: "I cannot verify these identifiers" and "these identifiers are invalid" are
different conditions, and reporting the second when the first is true would send an operator hunting a
configuration bug that does not exist. The catalog gate also substitutes the catalog's own spelling of
an identifier, so a case difference between the authored blob and the database resolves rather than
failing.

## Browse

`SqlDriverBrowser` browses the live catalog — schema → table → column — so the `/raw` browse picker
can commit columns as tags. A committed leaf carries **schema, table and column**: a column name alone
cannot address a SQL tag, and an earlier version that emitted only the column produced tags whose
typed editor opened empty.

Browse works against all three constructed providers; the catalog query is the dialect's own
(`INFORMATION_SCHEMA` on SQL Server and Postgres — with `pg_catalog` / `information_schema`
themselves excluded — and the `ALL_*` views on Oracle).

### The committed row selector (Gitea #552)

A browse-committed column becomes a **WideRow** tag, and a wide-row tag must say *which row* it reads
— the table and column alone are not a binding. The browse session therefore states a fourth address
field alongside schema/table/column: the table's preferred **date/time-typed** column, which the
commit writes as `rowSelector.topByTimestamp` ("read the newest row"). Before this the commit emitted
a selector-less blob that `SqlEquipmentTagParser` and the authoring gate both refuse, so no
browse-committed Sql tag was ever readable.

Three properties of the choice are deliberate:

- **Candidacy is by declared TYPE, never by column name.** The session asks
  `ISqlDialect.MapColumnType` for `DriverDataType.DateTime` — the same metadata the browse tree already
  renders per column. A name heuristic (`*_ts`, `*time*`) would happily order a wide-row read by a
  *string* column called `timestamp`, binding the tag to whichever row sorts last lexically.
- **Candidates are RANKED: a time-bearing type beats a date-only one.** `MapColumnType` reports
  `DriverDataType.DateTime` for SQL Server's and PostgreSQL's `date` too, and ordering a single-row read
  by a date-only column ties for *every row written on the same day* — "the newest row" quietly becomes
  storage order among the ties, and the reader cannot warn about it because the server has already
  discarded the others. So a `datetime2`/`timestamptz`/… column wins over a `date` one wherever both
  exist, regardless of declaration order. A date-only column is still stated when it is **all** the table
  has, so such a table stays bindable by browse rather than becoming un-committable.
  **Oracle's `DATE` is exempt** — it carries a time-of-day component (which is why `OracleDialect` maps
  it faithfully rather than as a widening), so it is not demoted there.
- **The tie-break within a rank is first in catalog order** (every dialect's column query orders by
  ordinal position), so the pick is stable across calls, connections and servers. On a
  `CreatedUtc`/`ModifiedUtc` pair it takes the earlier-declared one. It is a **visible, editable
  default**: the typed editor shows the selector and the operator can change it — to the other
  timestamp, or to a `whereColumn`/`whereValue` pair entirely.

**A table with no date/time column cannot be bound by browse, and the commit says so.** The remaining
selector is a `whereColumn`/`whereValue` pair whose value is *data*, not schema, so nothing in a
catalog can supply it. `RawBrowseCommitMapper.DescribeUncommittableLeaf` refuses that leaf by name,
names the table, and points at the tag editor; no selector is invented, and the mapper's blob stays
selector-less rather than dropping to a generic `{"address": …}` the typed editor cannot read.

## Authoring in the AdminUI

- **Driver config** — `/raw` → driver node → *Configure*. Sql is a **single-connection** driver: the
  connection lives on the driver, not on a device, and the modal says so.
- **Tags** — the typed `SqlTagConfigModel` editor, or CSV import (Sql has typed CSV columns), or
  browse-commit from the picker.

## Capabilities

`IDriver`, `ITagDiscovery`, `IReadable`, `ISubscribable`, `IHostConnectivityProbe`.

No `IWritable` (structural, above). No `IRediscoverable` — a SQL schema change is not signalled, and
`DiscoverAsync` echoes authored config rather than enumerating the database, so this driver is
deliberately excluded from any discovery-diff machinery.

## See also

- [`docs/Raw.md`](../Raw.md) — the `/raw` authoring tree
- [`docs/drivers/README.md`](README.md) — the driver index
- Gitea **#496** / **#497** / **#498** — the shipped follow-ups