postgres_syntax

Development

Consult PostgreSQL's parser and grammar (gram.y) to understand SQL syntax, DDL statement structure, and parsing rules when implementing pgschema features. Use this skill when generating DDL in internal/diff/*.go, validating SQL syntax, understanding keyword precedence, or learning how PostgreSQL handles specific constructs like triggers, indexes, generated columns, or constraint triggers.

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PostgreSQL Syntax Reference

Reference PostgreSQL's grammar to understand SQL syntax and generate correct DDL.

Source Files

Local copies (preferred):

internal/gram.y - Yacc/Bison grammar defining all PostgreSQL SQL syntax
internal/scan.l - Flex lexer for tokenization

Searching the grammar:

grep -n "CreateTrigStmt:" internal/gram.y     # Find statement rule
grep -A 10 "TriggerWhen:" internal/gram.y     # Understand an option

Statement Types → Grammar Rules

Statement	Grammar Rule	Key Sub-rules
CREATE TABLE	`CreateStmt`	`columnDef`, `TableConstraint`, `TableLikeClause`
ALTER TABLE	`AlterTableStmt`	`alter_table_cmd`
CREATE INDEX	`IndexStmt`	`index_elem` (column, function, expression)
CREATE TRIGGER	`CreateTrigStmt`	`TriggerActionTime`, `TriggerEvents`, `TriggerWhen`
CREATE FUNCTION	`CreateFunctionStmt`	`func_args`, `createfunc_opt_list`
CREATE VIEW	`ViewStmt`	`SelectStmt`
CREATE SEQUENCE	`CreateSeqStmt`	`OptSeqOptList`
CREATE TYPE	`CreateEnumStmt`, `CompositeTypeStmt`, `CreateDomainStmt`
CREATE POLICY	`CreatePolicyStmt`	`row_security_cmd`

Grammar Syntax Guide

gram.y uses Yacc/Bison notation:

UPPERCASE: Terminal tokens (keywords like CREATE, TRIGGER)
lowercase: Non-terminal rules (references to other grammar rules)
|: Alternative syntax options
opt_*: Optional elements (can be empty)
*_list: Recursive list constructs

Example:

CreateTrigStmt:
    CREATE opt_or_replace TRIGGER name TriggerActionTime TriggerEvents ON
    qualified_name TriggerReferencing TriggerForSpec TriggerWhen
    EXECUTE FUNCTION_or_PROCEDURE func_name '(' TriggerFuncArgs ')'

Key Constructs for pgschema

Column Definitions

Regular: column_name type [constraints]
Generated: column_name type GENERATED ALWAYS AS (expr) STORED
Identity: column_name type GENERATED {ALWAYS|BY DEFAULT} AS IDENTITY

Index Elements

Three forms — note extra parens for arbitrary expressions:

Column: CREATE INDEX idx ON t (col)
Function: CREATE INDEX idx ON t (lower(col))
Expression: CREATE INDEX idx ON t ((col + 1))

Trigger WHEN Clause

TriggerWhen:
    WHEN '(' a_expr ')'
    | /* EMPTY */

Constraint Triggers

CREATE opt_or_replace CONSTRAINT TRIGGER name ...
    -- Can be DEFERRABLE / NOT DEFERRABLE
    -- Can be INITIALLY DEFERRED / INITIALLY IMMEDIATE

Table LIKE Clause

LIKE qualified_name [INCLUDING|EXCLUDING] {COMMENTS|CONSTRAINTS|DEFAULTS|IDENTITY|GENERATED|INDEXES|STATISTICS|STORAGE|ALL}

Operator Precedence (from gram.y top)

%left OR
%left AND
%right NOT
%nonassoc IS ISNULL NOTNULL
%nonassoc '<' '>' '=' LESS_EQUALS GREATER_EQUALS NOT_EQUALS

Keywords

Reserved: Cannot be identifiers without quoting (SELECT, TABLE, CREATE)
Unreserved: Can be used as identifiers (ABORT, ACCESS, ACTION)

When generating DDL, quote identifiers that match reserved keywords.

Version Differences (14-18)

PG 14: COMPRESSION clause for tables
PG 15: UNIQUE NULLS NOT DISTINCT
PG 16: SQL/JSON functions
PG 17: MERGE enhancements

Check gram.y git history to see when features were added. Add version detection in pgschema if needed.

Applying to pgschema

When generating DDL in internal/diff/*.go:

Follow gram.y syntax exactly for keyword ordering
Include all required elements
Quote identifiers correctly via ir/quote.go
Test generated DDL against real PostgreSQL via integration tests