Edit syntax diagrams

How to edit syntax diagrams

This document describes how to make changes to YSQL API syntax diagrams or add new ones.

The following describes how to create and edit the syntax grammar and diagrams for the YSQL documentation. The YCQL documentation uses a different method. Take advice from colleagues if you need to work on YCQL diagrams.


Before editing syntax diagrams, you should understand some basic terminology.

Syntax rule

A syntax rule is the formal definition of the grammar of a SQL statement, or a component of a SQL statement.

Every syntax rule is defined textually in the single diagram definition file.

A syntax rule is specified using EBNF notation.

The following is an example of the syntax rule for the PREPARE statement:

prepare_statement ::= 'PREPARE' name [ '(' data_type { ',' data_type } ')' ] 'AS' statement ;
|-- Rule Name --|     |-- Rule Definition -->

When this is presented on the Grammar tab in the syntax diagram, it is transformed into the PostgreSQL notation. This is widely used in database documentation. But it is not suitable as input for a program that generate diagrams.

A syntax rule has two parts:

  • Rule name - This is left of ::= in the rule. Names conventionally use lower case with underscores.

  • Rule definition - This is right of ::= in the rule. The definition has three kinds of element:

    • EBNF syntax elements like these: [ ] ( ) ' { } ,

    • Keywords and punctuation that are part of the grammar that is being defined. Keywords are conventionally spelled in upper case. Each individual keyword is surrounded by single quotes. This conveys the meaning that whitespace between these, and other elements in the target grammar, is insignificant. Punctuation characters in the target grammar are surrounded by single quotes to distinguish them from punctuation characters in the EBNF grammar.

    • References to other rule names. These become clickable links in the generated diagrams (but not in the generated grammars). Rule names are spelled in lower case. Notice how underscore is used between the individual English words.

    The single space before the ; terminator is significant.

Syntax diagram

The result, in the published documentation, of a syntax rule that is contained in the diagram definition file.

The syntax diagram and the syntax rule bear a one-to-one mutual relationship.

The Syntax section of the account of the PREPARE statement provides a short, but sufficient, example.

The syntax diagram appears as a (member of a) tabbed pair which gives the reader the choice to see a syntax rule as either the "Grammar" form (in the syntax used in the PostgreSQL documentation and the documentation for several other databases) or the "Diagram" form (a so-called "railroad diagram" that again is used commonly in the documentation for several other databases).

Syntax diagram set

A set of syntax rule names grouped and displayed together on a content page.

Diagram definition file

The ysql_grammar.ebnf file located at /docs/content/preview/api/ysql/syntax_resources/ holds the definition, written in EBNF notation, of every syntax rule. This file in all its entirety is manually typed.

You create a syntax rule by adding it to this file.

The order in which the rules are specified in this file is reproduced in the grammar diagrams file. This, in turn, reflects decisions made by authors about what makes a convenient reading order. Try to spot what informs the present ordering and insert new rules in a way that respects this. In the limit, insert a new rule at the end as the last new properly-defined rule and before this comment:

(* Supporting rules *)

Grammar diagrams file

The grammar diagrams file contains every syntax diagram that is generated from all of the syntax rules that are found in the diagram definition file.

This file is automatically generated.

Add or modify syntax diagrams

Syntax diagrams are generated dynamically. The diagram/grammar pair are automatically populated based on the syntax rules.

There are typically two workflows:

  1. Add or modify syntax rules.

    You do this by editing the diagram definition file.

  2. Specify the syntax diagrams to be displayed on a page.

    You do this by adding an ebnf block to the page.

Add and modify syntax rules

To add a new syntax rule or modify an existing one, edit the diagram definition file.

If you are developing on a local machine, you need to restart Hugo after modifying the diagram definition file for the changes to be reflected on the local preview. Note that regenerating diagrams can take upwards of a minute or more, and longer if there are errors.

Check the syntax of your rule carefully; refer to Caveats for examples of errors you can encounter.

Specify syntax diagrams to be displayed on a page

To specify the syntax diagrams to display on a page, use the ebnf shortcode and provide the rule name of each syntax rule that you want included as a separate line ending with a comma.

For example, for window function invocations, to show select_start and window_clause on the page, you would add the following syntax diagram set:


This would add the grammar and syntax tabs as follows:

select_start ::= SELECT [ ALL | 
                          DISTINCT [ ON { ( expression [ , ... ] ) } ] ] 
                  [ * | { { expression
                            | fn_over_window
                            | ordinary_aggregate_fn_invocation
                            | within_group_aggregate_fn_invocation } 
                        [ [ AS ] name ] } [ , ... ] ]

window_clause ::= WINDOW { { name AS window_definition } [ , ... ] }

The syntax and grammar diagrams are generated in the same order as included in the ebnf shortcode.

Suppose that a syntax rule includes a reference to another syntax rule. If the referenced syntax rule is included in the same syntax diagram set, then the name of the syntax rule in the referring syntax diagram becomes a link to the syntax rule in that same syntax diagram set. Otherwise the generated link target of the referring rule is in the grammar diagrams file. The way that this link is spelled depends on the location of the .md file that includes the generated syntax diagram.

In the case you have multiple syntax diagram sets on the same page and would like to cross-reference each other on the same page, specify the local rules that need to be cross referenced as comma separated values in the localrefs argument of the ebnf shortcode. For example,

{{%ebnf localrefs="window_definition,frame_clause"%}}

This will ensure that any reference to window_definition or frame_clause in this syntax diagram set will link to another syntax diagram set on the same page and not to the grammar diagrams file.


Redundant use of ( ... )

In at least one case, the use of semantically redundant notation in the diagram definition file conveys the author's intent about how the diagram should be constructed. Here's an example of semantically significant use of ( ... ) in algebra:

  (a/b) - c differs in meaning from a/(b - c)

And while the rules do give a/b - c an unambiguous meaning (advertised by the absence of space surrounding the / operator), it's generally considered unhelpful to write this, thereby forcing the reader to rehearse the rules in order to discern the meaning. Rather, the semantically redundant orthography (a/b) - c is preferred.

In EBNF, ( ... ) plays a similar role as it does in algebra. In some situations, just which subset of items is grouped changes the meaning. But in some situations (like a + b + c in algebra) the use of ( ... ) conveys no semantics. Look for this at the end of the diagram definition file:

  Notice that the "demo-2" rule uses ( ... ) redundantly.
  The two rules are semantically identical but they produce the different diagrams.
  The diagrams do express the same meaning but they are drawn using different conventions.
  Uncomment the two rule definitions and look at the end of the grammar diagrams file to
  see the two semantically equivalent, but differently drawn, diagrams.
  Make sure that you comment these out again before creating a Pull request.

 demo-1-irrelevant-for-ysql-syntax ::= ( a 'K' b { ',' a 'K' b } ) ;
 demo-2-irrelevant-for-ysql-syntax ::= ( ( a 'K' b ) { ',' ( a 'K' b ) } ) ;

Missing the single-quotes around a keyword

Look for this syntax rule in the diagram definition file:

declare = 'DECLARE' cursor_name [ 'BINARY' ] [ 'INSENSITIVE' ] [ [ 'NO' ] 'SCROLL' ] \
          'CURSOR' [ ( 'WITH' | 'WITHOUT' ) 'HOLD' ] 'FOR' subquery ;

Remove the single quotes that surround 'INSENSITIVE'. This will change its status in EBNF's grammar from keyword to syntax rule. Most people find it hard to spot such a typo just by proofreading. Now restart Hugo. You won't see any errors reported on stderr. But if you look carefully at the stdout report, you'll see this warning:

WARNING: Undefined rules referenced in rule 'declare': [INSENSITIVE]

It's clear what it means. And you must fix it because otherwise a reader of the YSQL documentation will see a semantic error—and will wonder what on earth the insensitive rule is. Fix the problem immediately and restart Hugo. Then see a clean report again. This exercise tells you that it's a very good plan to restart Hugo after every edit to any single grammar rule. You'll know what rule you just edited and so you'll immediately know where to look for the error.

Misspelling a syntax rule

Look for this in the diagram definition file:

savepoint_rollback = ( 'ROLLBACK' ['WORK' | 'TRANSACTION' ] 'TO' [ 'SAVEPOINT' ] name ) ;

Edit it to change, say, the first ] character to }. It's easy to do this typo because these two characters are on the same key and it's hard to see the difference when you use a small font. Now restart Hugo. You'll see this on stderr:

Exception in thread "main" java.lang.IllegalStateException: This element must not be nested and should have been processed before entering generation.
  at net.nextencia.rrdiagram.grammar.rrdiagram.RRBreak.computeLayoutInfo(RRBreak.java:19)
  at net.nextencia.rrdiagram.grammar.rrdiagram.RRSequence.computeLayoutInfo(RRSequence.java:34)
  at net.nextencia.rrdiagram.grammar.rrdiagram.RRChoice.computeLayoutInfo(RRChoice.java:30)
  at net.nextencia.rrdiagram.grammar.rrdiagram.RRSequence.computeLayoutInfo(RRSequence.java:34)
  at net.nextencia.rrdiagram.grammar.rrdiagram.RRDiagram.toSVG(RRDiagram.java:333)
  at net.nextencia.rrdiagram.grammar.rrdiagram.RRDiagramToSVG.convert(RRDiagramToSVG.java:30)
  at net.nextencia.rrdiagram.Main.regenerateReferenceFile(Main.java:139)
  at net.nextencia.rrdiagram.Main.regenerateFolder(Main.java:72)
  at net.nextencia.rrdiagram.Main.main(Main.java:54)

The error will cause the notorious Hugo black screen in the browser. It's best to <ctrl>-C Hugo now.

This is hardly user-friendly! You'll also see several warnings on stdout. Almost all of these are basic consequences of the actual problem and so tell you nothing. Here's the significant information:

WARNING: Exception occurred while exporting rule savepoint_rollback
WARNING: savepoint_rollback = 'ROLLBACK' [ 'WORK' | 'TRANSACTION' } 'TO' [ 'SAVEPOINT' ] name ) ; ...

You can see that the offending } is mentioned. Fix it immediately, restart Hugo, and then see a clean report again. This exercise, too, tells you that you should restart Hugo after every edit to a grammar rule. Here too, you'll know what rule you just edited and so will immediately know where to look for the error.