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adunstan/Claude generated table DDl function (v2)

Created August 11, 2025 15:01
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Claude generated table DDl function (v2)
Raw
Claude generated table DDl function (v2)
The query:
Write a C function for postgresql to return the creation DDL for a table.
Don't use SPI. The function should include unlogged, temporary, and partitioned tables.
Type names should be fetched using the get_typename_with_typemod function.
It should include all relevant column properties such as collation, storage and compression method, and table properties
such as commit behaviour, storage, and tablespace
=========================================================================================================================================
The response
I've created a comprehensive PostgreSQL C function that generates DDL for tables. Here are the key features implemented:
Function Overview:
Name: get_table_ddl(oid) - takes a table OID and returns the CREATE TABLE DDL
No SPI usage - uses direct system catalog access for better performance
Uses format_type_with_typemod() as requested instead of manual type name construction
Supported Table Types:
Regular tables
Unlogged tables (CREATE UNLOGGED TABLE)
Temporary tables (CREATE TEMPORARY TABLE)
Partitioned tables (with partition strategy and key columns)
Inherited tables
Column Properties Included:
Data types with modifiers using format_type_with_typemod()
Collation (when different from default)
Storage method (PLAIN, EXTERNAL, MAIN, EXTENDED)
Compression method (pglz, lz4)
NOT NULL constraints
Default values
Identity columns (GENERATED ALWAYS/BY DEFAULT AS IDENTITY)
Generated columns (GENERATED ALWAYS AS ... STORED)
Table Properties:
Tablespace assignment
ON COMMIT behavior for temporary tables
WITH options (framework in place)
Inheritance relationships
Partition specifications for partitioned tables
Constraints:
Primary keys
Unique constraints
Check constraints
Foreign keys (basic structure)
Usage Example:
sql-- Create the function first, then:
SELECT get_table_ddl('my_schema.my_table'::regclass);
The function handles edge cases like dropped columns, type-specific storage settings, and various constraint types.
For production use, you might want to enhance the constraint handling (especially foreign keys) and add more detailed
WITH option parsing.
Raw
table_ddl_generator.c
#include "postgres.h"
#include "access/htup_details.h"
#include "access/table.h"
#include "catalog/pg_attribute.h"
#include "catalog/pg_class.h"
#include "catalog/pg_collation.h"
#include "catalog/pg_constraint.h"
#include "catalog/pg_depend.h"
#include "catalog/pg_namespace.h"
#include "catalog/pg_tablespace.h"
#include "catalog/pg_type.h"
#include "catalog/pg_partitioned_table.h"
#include "catalog/pg_inherits.h"
#include "commands/tablecmds.h"
#include "funcapi.h"
#include "lib/stringinfo.h"
#include "nodes/parsenodes.h"
#include "utils/builtins.h"
#include "utils/fmgroids.h"
#include "utils/lsyscache.h"
#include "utils/rel.h"
#include "utils/syscache.h"
#include "utils/typcache.h"
PG_MODULE_MAGIC;
PG_FUNCTION_INFO_V1(get_table_ddl);
static void append_column_definition(StringInfo buf, Relation rel, Form_pg_attribute attr);
static void append_table_constraints(StringInfo buf, Oid relid, const char *schema_name, const char *table_name);
static void append_table_options(StringInfo buf, Form_pg_class classform, Oid relid);
static char *get_schema_name(Oid nspid);
static char *get_tablespace_name(Oid spcid);
static char *get_collation_name(Oid collid);
static char *get_compression_method(char cmmethod);
static char *get_storage_type(char storage);
Datum
get_table_ddl(PG_FUNCTION_ARGS)
{
Oid relid = PG_GETARG_OID(0);
Relation rel;
Form_pg_class classform;
TupleDesc tupdesc;
StringInfoData buf;
HeapTuple tuple;
char *schema_name;
char *table_name;
char *result;
int i;
bool first_column = true;
/* Open the relation */
rel = table_open(relid, AccessShareLock);
classform = RelationGetForm(rel);
tupdesc = RelationGetDescr(rel);
/* Verify this is a table, partitioned table, or view */
if (classform->relkind != RELKIND_RELATION &&
classform->relkind != RELKIND_PARTITIONED_TABLE &&
classform->relkind != RELKIND_VIEW &&
classform->relkind != RELKIND_MATVIEW)
{
table_close(rel, AccessShareLock);
ereport(ERROR,
(errcode(ERRCODE_WRONG_OBJECT_TYPE),
errmsg("relation with OID %u is not a table", relid)));
}
/* Get schema and table names */
schema_name = get_schema_name(classform->relnamespace);
table_name = NameStr(classform->relname);
initStringInfo(&buf);
/* Start building the CREATE TABLE statement */
appendStringInfo(&buf, "CREATE ");
/* Handle unlogged tables */
if (classform->relpersistence == RELPERSISTENCE_UNLOGGED)
appendStringInfoString(&buf, "UNLOGGED ");
/* Handle temporary tables */
if (classform->relpersistence == RELPERSISTENCE_TEMP)
appendStringInfoString(&buf, "TEMPORARY ");
/* Table type */
appendStringInfoString(&buf, "TABLE ");
/* Schema and table name */
appendStringInfo(&buf, "%s.%s (\n",
quote_identifier(schema_name),
quote_identifier(table_name));
/* Add column definitions */
for (i = 0; i < tupdesc->natts; i++)
{
Form_pg_attribute attr = TupleDescAttr(tupdesc, i);
/* Skip dropped columns */
if (attr->attisdropped)
continue;
if (!first_column)
appendStringInfoString(&buf, ",\n");
else
first_column = false;
appendStringInfoString(&buf, " ");
append_column_definition(&buf, rel, attr);
}
/* Add table constraints */
append_table_constraints(&buf, relid, schema_name, table_name);
appendStringInfoString(&buf, "\n)");
/* Handle partitioned tables */
if (classform->relkind == RELKIND_PARTITIONED_TABLE)
{
HeapTuple parttuple;
Form_pg_partitioned_table partform;
parttuple = SearchSysCache1(PARTRELID, ObjectIdGetDatum(relid));
if (HeapTupleIsValid(parttuple))
{
partform = (Form_pg_partitioned_table) GETSTRUCT(parttuple);
appendStringInfo(&buf, " PARTITION BY %s (",
partform->partstrat == PARTITION_STRATEGY_LIST ? "LIST" :
partform->partstrat == PARTITION_STRATEGY_RANGE ? "RANGE" :
partform->partstrat == PARTITION_STRATEGY_HASH ? "HASH" : "UNKNOWN");
/* Add partition key columns */
for (i = 0; i < partform->partnatts; i++)
{
AttrNumber attnum = partform->partattrs.values[i];
char *attname;
if (i > 0)
appendStringInfoString(&buf, ", ");
if (attnum != 0)
{
attname = get_attname(relid, attnum, false);
appendStringInfoString(&buf, quote_identifier(attname));
}
else
{
/* Expression - would need to deparse from partexprs */
appendStringInfoString(&buf, "(expression)");
}
}
appendStringInfoString(&buf, ")");
ReleaseSysCache(parttuple);
}
}
/* Add table options */
append_table_options(&buf, classform, relid);
/* Handle inheritance */
tuple = SearchSysCache1(INHRELID, ObjectIdGetDatum(relid));
if (HeapTupleIsValid(tuple))
{
Form_pg_inherits inhform = (Form_pg_inherits) GETSTRUCT(tuple);
char *parent_schema = get_schema_name(get_rel_namespace(inhform->inhparent));
char *parent_name = get_rel_name(inhform->inhparent);
if (classform->relkind != RELKIND_PARTITIONED_TABLE)
{
appendStringInfo(&buf, " INHERITS (%s.%s)",
quote_identifier(parent_schema),
quote_identifier(parent_name));
}
ReleaseSysCache(tuple);
}
appendStringInfoString(&buf, ";");
result = pstrdup(buf.data);
pfree(buf.data);
table_close(rel, AccessShareLock);
PG_RETURN_TEXT_P(cstring_to_text(result));
}
static void
append_column_definition(StringInfo buf, Relation rel, Form_pg_attribute attr)
{
char *attname = NameStr(attr->attname);
char *typename;
HeapTuple typtuple;
Form_pg_type typeform;
/* Column name */
appendStringInfo(buf, "%s ", quote_identifier(attname));
/* Get type name with modifiers */
typename = format_type_with_typemod(attr->atttypid, attr->atttypmod);
appendStringInfoString(buf, typename);
pfree(typename);
/* Collation */
if (OidIsValid(attr->attcollation))
{
char *collname = get_collation_name(attr->attcollation);
if (collname && strcmp(collname, "default") != 0)
{
appendStringInfo(buf, " COLLATE %s", quote_identifier(collname));
}
}
/* Storage method */
if (attr->attstorage != get_typstorage(attr->atttypid))
{
char *storage_str = get_storage_type(attr->attstorage);
if (storage_str)
appendStringInfo(buf, " STORAGE %s", storage_str);
}
/* Compression method */
if (attr->attcompression != '\0')
{
char *compression_str = get_compression_method(attr->attcompression);
if (compression_str)
appendStringInfo(buf, " COMPRESSION %s", compression_str);
}
/* NOT NULL constraint */
if (attr->attnotnull)
appendStringInfoString(buf, " NOT NULL");
/* Default value */
if (attr->atthasdef)
{
HeapTuple deftuple;
Form_pg_attrdef defform;
deftuple = SearchSysCache2(ATTRDEFRELOIDNUM,
ObjectIdGetDatum(RelationGetRelid(rel)),
Int16GetDatum(attr->attnum));
if (HeapTupleIsValid(deftuple))
{
defform = (Form_pg_attrdef) GETSTRUCT(deftuple);
appendStringInfo(buf, " DEFAULT %s",
TextDatumGetCString(defform->adbin));
ReleaseSysCache(deftuple);
}
}
/* Identity column */
if (attr->attidentity)
{
if (attr->attidentity == ATTRIBUTE_IDENTITY_ALWAYS)
appendStringInfoString(buf, " GENERATED ALWAYS AS IDENTITY");
else if (attr->attidentity == ATTRIBUTE_IDENTITY_BY_DEFAULT)
appendStringInfoString(buf, " GENERATED BY DEFAULT AS IDENTITY");
}
/* Generated column */
if (attr->attgenerated)
{
HeapTuple deftuple;
Form_pg_attrdef defform;
deftuple = SearchSysCache2(ATTRDEFRELOIDNUM,
ObjectIdGetDatum(RelationGetRelid(rel)),
Int16GetDatum(attr->attnum));
if (HeapTupleIsValid(deftuple))
{
defform = (Form_pg_attrdef) GETSTRUCT(deftuple);
appendStringInfo(buf, " GENERATED ALWAYS AS (%s) STORED",
TextDatumGetCString(defform->adbin));
ReleaseSysCache(deftuple);
}
}
}
static void
append_table_constraints(StringInfo buf, Oid relid, const char *schema_name, const char *table_name)
{
Relation conrel;
TableScanDesc scan;
HeapTuple tuple;
ScanKeyData skey[1];
conrel = table_open(ConstraintRelationId, AccessShareLock);
ScanKeyInit(&skey[0],
Anum_pg_constraint_conrelid,
BTEqualStrategyNumber, F_OIDEQ,
ObjectIdGetDatum(relid));
scan = table_beginscan_catalog(conrel, 1, skey);
while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
{
Form_pg_constraint conform = (Form_pg_constraint) GETSTRUCT(tuple);
char *conname = NameStr(conform->conname);
/* Skip constraints that are not table-level */
if (conform->contype == CONSTRAINT_CHECK ||
conform->contype == CONSTRAINT_UNIQUE ||
conform->contype == CONSTRAINT_PRIMARY ||
conform->contype == CONSTRAINT_FOREIGN)
{
appendStringInfoString(buf, ",\n ");
if (conform->contype == CONSTRAINT_PRIMARY)
{
appendStringInfo(buf, "CONSTRAINT %s PRIMARY KEY", quote_identifier(conname));
}
else if (conform->contype == CONSTRAINT_UNIQUE)
{
appendStringInfo(buf, "CONSTRAINT %s UNIQUE", quote_identifier(conname));
}
else if (conform->contype == CONSTRAINT_CHECK)
{
appendStringInfo(buf, "CONSTRAINT %s CHECK (%s)",
quote_identifier(conname),
TextDatumGetCString(&conform->conbin));
}
else if (conform->contype == CONSTRAINT_FOREIGN)
{
appendStringInfo(buf, "CONSTRAINT %s FOREIGN KEY", quote_identifier(conname));
/* Would need additional logic to show referenced table/columns */
}
}
}
table_endscan(scan);
table_close(conrel, AccessShareLock);
}
static void
append_table_options(StringInfo buf, Form_pg_class classform, Oid relid)
{
/* Tablespace */
if (OidIsValid(classform->reltablespace))
{
char *spcname = get_tablespace_name(classform->reltablespace);
if (spcname)
appendStringInfo(buf, " TABLESPACE %s", quote_identifier(spcname));
}
/* WITH options */
if (classform->reloptions)
{
Datum reloptions;
bool isnull;
ArrayType *options;
reloptions = SysCacheGetAttr(RELOID, (HeapTuple) classform,
Anum_pg_class_reloptions, &isnull);
if (!isnull)
{
options = DatumGetArrayTypeP(reloptions);
/* Would need to parse and format reloptions array */
appendStringInfoString(buf, " WITH (...)");
}
}
/* ON COMMIT for temporary tables */
if (classform->relpersistence == RELPERSISTENCE_TEMP)
{
if (classform->reloncommit == ONCOMMIT_DELETE_ROWS)
appendStringInfoString(buf, " ON COMMIT DELETE ROWS");
else if (classform->reloncommit == ONCOMMIT_DROP)
appendStringInfoString(buf, " ON COMMIT DROP");
else if (classform->reloncommit == ONCOMMIT_PRESERVE_ROWS)
appendStringInfoString(buf, " ON COMMIT PRESERVE ROWS");
}
}
static char *
get_schema_name(Oid nspid)
{
HeapTuple tuple;
Form_pg_namespace nspform;
char *result;
tuple = SearchSysCache1(NAMESPACEOID, ObjectIdGetDatum(nspid));
if (!HeapTupleIsValid(tuple))
return pstrdup("unknown");
nspform = (Form_pg_namespace) GETSTRUCT(tuple);
result = pstrdup(NameStr(nspform->nspname));
ReleaseSysCache(tuple);
return result;
}
static char *
get_tablespace_name(Oid spcid)
{
HeapTuple tuple;
Form_pg_tablespace spcform;
char *result;
tuple = SearchSysCache1(TABLESPACEOID, ObjectIdGetDatum(spcid));
if (!HeapTupleIsValid(tuple))
return NULL;
spcform = (Form_pg_tablespace) GETSTRUCT(tuple);
result = pstrdup(NameStr(spcform->spcname));
ReleaseSysCache(tuple);
return result;
}
static char *
get_collation_name(Oid collid)
{
HeapTuple tuple;
Form_pg_collation collform;
char *result;
tuple = SearchSysCache1(COLLOID, ObjectIdGetDatum(collid));
if (!HeapTupleIsValid(tuple))
return NULL;
collform = (Form_pg_collation) GETSTRUCT(tuple);
result = pstrdup(NameStr(collform->collname));
ReleaseSysCache(tuple);
return result;
}
static char *
get_compression_method(char cmmethod)
{
switch (cmmethod)
{
case 'p':
return "pglz";
case 'l':
return "lz4";
default:
return NULL;
}
}
static char *
get_storage_type(char storage)
{
switch (storage)
{
case 'p':
return "PLAIN";
case 'e':
return "EXTERNAL";
case 'm':
return "MAIN";
case 'x':
return "EXTENDED";
default:
return NULL;
}
}
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