Module read 

The read model: a program’s maintained state as relations, and a small SQL over them.

docs/05-tier-lowering.md §5.3 names this as one of the four things the data tier owes:

Read models … one-shot queries and pgwire access for the outside world: psql, BI tools, DBeaver see materialized views as ordinary tables — the single cheapest trust-builder for adopting teams

What a read model is here, and what it is not

§5.3’s row also says “generated tables in the same Postgres”, and that is not what this builds. A read model is not a second copy of the state written on the append path; it is the collection the fold already holds and the arrangement crate::engine already maintains, projected. Three consequences, and they are the argument for it:

• A read model costs nothing per event. Nothing is written, nothing is projected, and the sequencer is untouched — which is 26 §26.2’s rule (“who advances it: not the sequencer”) applied to a second kind of reader rather than argued with.
• It cannot disagree with the page. A durable projection is a second code path, and a second code path over the same events is a thing that can drift. These rows are read from the same arrangement the view renders from, so the recompute oracle already covers them.
• It is exactly as fresh as the query. A query advances the dataflow to the log’s head and then reads, so a SELECT issued after an ack sees that ack’s event. There is no projection lag because there is no projection.

What that costs is the one-transaction property 67 §67.1 held open: an append and its projection are still not one transaction, because there is still no projection. §88.6 is the row-by-row list.

Where the tables come from

Table	Rows	Read from
a collection-valued field of the accumulator	its elements	the state value
the accumulator’s remaining scalar fields	one	the state value
a declared signal that does not read the session	its elements, or one	the maintained node

The third row is the interesting one: a read model is a view that does not depend on who is asking, which is the same cut §5.3 draws for arrangement sharing. A per_session signal is not a table because a SQL client is not a session — it has no Session to be rendered for, and inventing one would answer a question nobody asked.

What this is not

It is not a query planner. 04 §4.2 keeps the Query sub-language symbolic, and §20.5 holds beck explain query until an engine compiles one; what parse accepts is a hand-written subset over one table at a time, with no joins, no subqueries and no aggregation beyond count(*). It exists so that an outside tool can read what the program holds, which is what §5.3’s row is for.

Structs

Answer

What a query answered.

Column

Cond

Schema

Every table a program’s read model has.

Select

What a query asks for.

SqlError

Why a query could not be answered. The message reaches the client verbatim.

Table

Enums

Cardinality

How many rows a table can have, which is a fact about its shape rather than about its data.

CmpOp

Datum

Item

Source

Where a table’s rows are read from.

SqlTy

The four SQL types a Beck scalar maps onto.

Stmt

A statement, which is a select or one of the two things a client says before it asks for anything.

Traits

Rows

Where a table’s rows come from. Implemented by whoever holds the running program.

Functions

at_path

Follow a path of field names into a value.

elements

The elements of a collection value, in the order it holds them.

parse

Parse one statement.

quote_ident

A name as it has to be written in this SQL: bare when it can be, quoted when it cannot.

version

What select version() answers.

Type Aliases

Cell

A value in one column, or SQL NULL.