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28 changes: 16 additions & 12 deletions _src/schema/basic.md
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Expand Up @@ -25,30 +25,34 @@ facts. Each fact is an instance of one predicate.
A predicate looks like this:

```
predicate P : KeyType -> ValueType
predicate P : KeyType
```

where

* `P` is the name of the predicate. e.g. `src.File`
* `KeyType` is the *key type*
* `ValueType` is the *value type*
* `KeyType` is the *key type*; the type of the facts.

`KeyType` and `ValueType` are types; the type language is described in
[Built-in Types](types.md).
The type language is described in [Built-in Types](types.md).

Every *fact* in predicate P has:

* A ***Fact ID***: a unique 64-bit integer that identifies the fact
* A ***Key***, a term of type `KeyType`
* A ***Value***, a term of type `ValueType`

(we use "term" here to avoid confusion with the use of "value" in a predicate).
A predicate can also have a value in addition to the key:

```
predicate P : KeyType -> ValueType
```

Often the value is empty, represented by the empty record `{}`. Most
predicates have all their information in the key.
facts of `P` would then have a fact ID, a key term and a value term.
When a predicate has a value in addition to the key, Glean enforces
the property that *each unique key has exactly one value*. It is
illegal to insert two facts with the same key and different values.

You can think of Glean as like a key/value store: we can look up a key
`K` in predicate `P` and get back value `V.` We can also query for
`K` in predicate `P` and get back value `V`. We can also query for
*patterns* that match multiple keys, and get back all the facts that
match the pattern. More about this when we talk about [Angle](../angle/intro.md)
queries.
match the pattern. More about this when we talk about
[Angle](../angle/intro.md) queries.
6 changes: 6 additions & 0 deletions _src/schema/design.md
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Expand Up @@ -137,6 +137,12 @@ If you're choosing between arrays and separate facts, then consider:
* Facts with large arrays are also slower to search through in a query
than smaller facts.

## Increase sharing

If there is duplication in the data stored in our facts, we can often
extract the common data into a predicate to increase sharing. One
example of this was described in [What is the difference between a predicate and a type?](schema/syntax.md#what-is-the-difference-between-a-predicate-and-a-type).

## How to experiment with schema design

* Generate some data and see how large it is, using `:stat` in the shell.
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55 changes: 47 additions & 8 deletions _src/schema/syntax.md
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Expand Up @@ -8,7 +8,10 @@ Schemas are written in Glean's **Angle** language.

## Named schemas

We usually create a new named schema for a family of related types and predicates. For example `hack` is the schema for facts about Hack code, `cxx` is the schema for facts about C/C++ and Objective C code, `java` is the schema for facts about Java code, and so on.
We usually create a new named schema for a family of related types and
predicates. For example `cxx` is the schema for facts about C/C++ and
Objective C code, `java` is the schema for facts about Java code,
`python` is the schema for facts about Python code and so on.

```
schema java.1 {
Expand All @@ -33,13 +36,12 @@ generated [Thrift file](thrift.md) for working with the data in your code.
A predicate definition looks like this:

```
predicate P : KeyType -> ValueType
predicate P : KeyType
```
where

* `P` is the name of the predicate. e.g. `src.File`
* `KeyType` is the *key type*
* `ValueType` is the *value type*

For example, the example schema that we saw in the [Introduction](../introduction.md)
contains the `Class` predicate:
Expand All @@ -60,6 +62,15 @@ This defines:
* a field `name`, that has type `string`
* a field `line`, of type `nat`

A predicate can also have a value:

```
predicate P : KeyType -> ValueType
```

We sometimes call these key-value predicates. Most predicates you
encounter will only have a key.

## Referring to other predicates

A key feature of Glean is the ability to refer directly to other types
Expand Down Expand Up @@ -138,14 +149,42 @@ type Loc = {
}
```

:::note
### What is the difference between a predicate and a type?

`type` declarations cannot be recursive or mutually recursive. If you want a
recursive type, the cycle must go through at least one predicate. For
more details, see [Recursion](recursion.md).
Predicates describe facts that are stored in the DB. A type has no
facts, it's just a name for a type. Types are expanded everywhere
they're mentioned.

:::
Predicates are useful for increasing sharing. For example, if we have

```
predicate P: { a : string, b : [nat] }
```

and we notice that many `P` facts have the same array of `nat` for the
`b` field, we could store the data more compactly by making it a predicate:

```
predicate List: [nat]
predicate P: { a : string, b : List }
```

Now there will only be one fact for each unique value of `List` stored
in the DB. Using a `type` would not have the same effect:

```
type List: [nat]
predicate P: { a : string, b : List }
```

This is exactly equivalent to the original version; every `P` fact
will contain the full `[nat]`.

Finally, `type` declarations cannot be recursive or mutually
recursive. If you want a recursive type, the cycle must go through at
least one predicate. For more details, see [Recursion](recursion.md).

## Importing and inheriting

Expand Down
4 changes: 1 addition & 3 deletions _src/schema/types.md
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Expand Up @@ -4,8 +4,6 @@ title: Built-in Types
sidebar_label: Built-in Types
---

## Built-in Types

| Type | Meaning |
|-------|-------|
| <code>nat</code> | 64-bit natural numbers |
Expand All @@ -15,6 +13,6 @@ sidebar_label: Built-in Types
| <code>{ field₁ : T₁, ..., fieldₙ : Tₙ }</code> | a record with zero or more named fields |
| <code>{ field₁ : T₁ &#124; ... &#124; fieldₙ : Tₙ }</code> | a sum (union) type with one or more named alternatives |
| <code>P</code> | a reference to a fact of predicate P |
| <code>bool</code> | the boolean type with values **true** or **false** |
| <code>bool</code> | the boolean type with values **true** and **false** |
| <code>maybe T</code> | an optional value of type T |
| <code>enum { name₁ &#124; ... &#124; nameₙ }</code> | exactly one of the symbols name₁..nameₙ |
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