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Enable fast default for domains with non-volatile constraints

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Previously, ALTER TABLE ADD COLUMN always forced a table rewrite when
the column type was a domain with constraints (CHECK or NOT NULL), even
if the default value satisfied those constraints.  This was because
contain_volatile_functions() considers CoerceToDomain immutable, so
the code conservatively assumed any constrained domain might fail.

Improve this by using soft error handling (ErrorSaveContext) to evaluate
the CoerceToDomain expression at ALTER TABLE time.  If the default value
passes the domain's constraints, the value is stored as a "missing"
attribute default and no table rewrite is needed.  If the constraint
check fails, we fall back to a table rewrite, preserving the historical
behavior that constraint violations are only raised when the table
actually contains rows.

Domains with volatile constraint expressions always require a table
rewrite since the constraint result could differ per evaluation and
cannot be cached.

Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Viktor Holmberg <viktor.holmberg@aiven.io>
Discussion: https://postgr.es/m/CACJufxE_+iZBR1i49k_AHigppPwLTJi6km8NOsC7FWvKdEmmXg@mail.gmail.com
This commit is contained in:
Andrew Dunstan 2026-03-12 17:53:09 -04:00
parent 487cf2cbd2
commit a0b6ef29a5
5 changed files with 190 additions and 18 deletions
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59
src/backend/commands/tablecmds.c
View file

@ -7486,15 +7486,6 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
* NULL if so, so without any modification of the tuple data we will get
* the effect of NULL values in the new column.
*
* An exception occurs when the new column is of a domain type: the domain
* might have a not-null constraint, or a check constraint that indirectly
* rejects nulls. If there are any domain constraints then we construct
* an explicit NULL default value that will be passed through
* CoerceToDomain processing. (This is a tad inefficient, since it causes
* rewriting the table which we really wouldn't have to do; but we do it
* to preserve the historical behavior that such a failure will be raised
* only if the table currently contains some rows.)
*
* Note: we use build_column_default, and not just the cooked default
* returned by AddRelationNewConstraints, so that the right thing happens
* when a datatype's default applies.
@ -7513,6 +7504,7 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
{
bool has_domain_constraints;
bool has_missing = false;
bool has_volatile = false;
/*
* For an identity column, we can't use build_column_default(),
@ -7530,8 +7522,18 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
else
defval = (Expr *) build_column_default(rel, attribute->attnum);
has_domain_constraints =
DomainHasConstraints(attribute->atttypid, &has_volatile);
/*
* If the domain has volatile constraints, we must do a table rewrite
* since the constraint result could differ per row and cannot be
* evaluated once and cached as a missing value.
*/
if (has_volatile)
tab->rewrite |= AT_REWRITE_DEFAULT_VAL;
/* Build CoerceToDomain(NULL) expression if needed */
has_domain_constraints = DomainHasConstraints(attribute->atttypid, NULL);
if (!defval && has_domain_constraints)
{
Oid baseTypeId;
@ -7573,27 +7575,50 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
* Attempt to skip a complete table rewrite by storing the
* specified DEFAULT value outside of the heap. This is only
* allowed for plain relations and non-generated columns, and the
* default expression can't be volatile (stable is OK). Note that
* contain_volatile_functions deems CoerceToDomain immutable, but
* here we consider that coercion to a domain with constraints is
* volatile; else it might fail even when the table is empty.
* default expression can't be volatile (stable is OK), and the
* domain constraint expressions can't be volatile (stable is OK).
*
* Note that contain_volatile_functions considers CoerceToDomain
* immutable, so we rely on DomainHasConstraints (called above)
* rather than checking defval alone.
*
* For domains with non-volatile constraints, we evaluate the
* default using soft error handling: if the constraint check
* fails (e.g., CHECK(value > 10) with DEFAULT 8), we fall back to
* a table rewrite. This preserves the historical behavior that
* such a failure is only raised when the table has rows.
*/
if (rel->rd_rel->relkind == RELKIND_RELATION &&
!colDef->generated &&
!has_domain_constraints &&
!has_volatile &&
!contain_volatile_functions((Node *) defval))
{
EState *estate;
ExprState *exprState;
Datum missingval;
bool missingIsNull;
ErrorSaveContext escontext = {T_ErrorSaveContext};
/* Evaluate the default expression */
/* Evaluate the default expression with soft errors */
estate = CreateExecutorState();
exprState = ExecPrepareExpr(defval, estate);
exprState = ExecPrepareExprWithContext(defval, estate,
(Node *) &escontext);
missingval = ExecEvalExpr(exprState,
GetPerTupleExprContext(estate),
&missingIsNull);
/*
* If the domain constraint check failed (via errsave),
* missingval is unreliable. Fall back to a table rewrite;
* Phase 3 will re-evaluate with hard errors, so the user gets
* an error only if the table has rows.
*/
if (SOFT_ERROR_OCCURRED(&escontext))
{
missingIsNull = true;
tab->rewrite |= AT_REWRITE_DEFAULT_VAL;
}
/* If it turns out NULL, nothing to do; else store it */
if (!missingIsNull)
{

35
src/backend/executor/execExpr.c
View file

@ -141,6 +141,26 @@ static void ExecInitJsonCoercion(ExprState *state, JsonReturning *returning,
*/
ExprState *
ExecInitExpr(Expr *node, PlanState *parent)
{
return ExecInitExprWithContext(node, parent, NULL);
}
/*
* ExecInitExprWithContext: same as ExecInitExpr, but with an optional
* ErrorSaveContext for soft error handling.
*
* When 'escontext' is non-NULL, expression nodes that support soft errors
* (currently CoerceToDomain's NOT NULL and CHECK constraint steps) will use
* errsave() instead of ereport(), allowing the caller to detect and handle
* failures without a transaction abort.
*
* The escontext must be provided at initialization time (not after), because
* it is copied into per-step data during expression compilation.
*
* Not all expression node types support soft errors. If in doubt, pass NULL.
*/
ExprState *
ExecInitExprWithContext(Expr *node, PlanState *parent, Node *escontext)
{
ExprState *state;
ExprEvalStep scratch = {0};
@ -154,6 +174,7 @@ ExecInitExpr(Expr *node, PlanState *parent)
state->expr = node;
state->parent = parent;
state->ext_params = NULL;
state->escontext = (ErrorSaveContext *) escontext;
/* Insert setup steps as needed */
ExecCreateExprSetupSteps(state, (Node *) node);
@ -763,6 +784,18 @@ ExecBuildUpdateProjection(List *targetList,
*/
ExprState *
ExecPrepareExpr(Expr *node, EState *estate)
{
return ExecPrepareExprWithContext(node, estate, NULL);
}
/*
* ExecPrepareExprWithContext: same as ExecPrepareExpr, but with an optional
* ErrorSaveContext for soft error handling.
*
* See ExecInitExprWithContext for details on the escontext parameter.
*/
ExprState *
ExecPrepareExprWithContext(Expr *node, EState *estate, Node *escontext)
{
ExprState *result;
MemoryContext oldcontext;
@ -771,7 +804,7 @@ ExecPrepareExpr(Expr *node, EState *estate)
node = expression_planner(node);
result = ExecInitExpr(node, NULL);
result = ExecInitExprWithContext(node, NULL, escontext);
MemoryContextSwitchTo(oldcontext);

2
src/include/executor/executor.h
View file

@ -324,6 +324,7 @@ ExecProcNode(PlanState *node)
* prototypes from functions in execExpr.c
*/
extern ExprState *ExecInitExpr(Expr *node, PlanState *parent);
extern ExprState *ExecInitExprWithContext(Expr *node, PlanState *parent, Node *escontext);
extern ExprState *ExecInitExprWithParams(Expr *node, ParamListInfo ext_params);
extern ExprState *ExecInitQual(List *qual, PlanState *parent);
extern ExprState *ExecInitCheck(List *qual, PlanState *parent);
@ -372,6 +373,7 @@ extern ProjectionInfo *ExecBuildUpdateProjection(List *targetList,
TupleTableSlot *slot,
PlanState *parent);
extern ExprState *ExecPrepareExpr(Expr *node, EState *estate);
extern ExprState *ExecPrepareExprWithContext(Expr *node, EState *estate, Node *escontext);
extern ExprState *ExecPrepareQual(List *qual, EState *estate);
extern ExprState *ExecPrepareCheck(List *qual, EState *estate);
extern List *ExecPrepareExprList(List *nodes, EState *estate);

61
src/test/regress/expected/fast_default.out
View file

@ -317,11 +317,72 @@ SELECT a, b, length(c) = 3 as c_ok, d, e >= 10 as e_ok FROM t2;
2 | 3 | t | {This,is,abcd,the,real,world} | t
(2 rows)
-- test fast default over domains with constraints
CREATE DOMAIN domain5 AS int CHECK(value > 10) DEFAULT 8;
CREATE DOMAIN domain6 as int CHECK(value > 10) DEFAULT random(min=>11, max=>100);
CREATE DOMAIN domain7 as int CHECK((value + random(min=>11::int, max=>11)) > 12);
CREATE DOMAIN domain8 as int NOT NULL;
CREATE TABLE test_add_domain_col(a int);
-- succeeds despite constraint-violating default because table is empty
ALTER TABLE test_add_domain_col ADD COLUMN a1 domain5;
NOTICE: rewriting table test_add_domain_col for reason 2
ALTER TABLE test_add_domain_col DROP COLUMN a1;
INSERT INTO test_add_domain_col VALUES(1),(2);
-- tests with non-empty table
ALTER TABLE test_add_domain_col ADD COLUMN b domain5; -- table rewrite, then fail
NOTICE: rewriting table test_add_domain_col for reason 2
ERROR: value for domain domain5 violates check constraint "domain5_check"
ALTER TABLE test_add_domain_col ADD COLUMN b domain8; -- table rewrite, then fail
NOTICE: rewriting table test_add_domain_col for reason 2
ERROR: domain domain8 does not allow null values
ALTER TABLE test_add_domain_col ADD COLUMN b domain5 DEFAULT 1; -- table rewrite, then fail
NOTICE: rewriting table test_add_domain_col for reason 2
ERROR: value for domain domain5 violates check constraint "domain5_check"
ALTER TABLE test_add_domain_col ADD COLUMN b domain5 DEFAULT 12; -- ok, no table rewrite
-- explicit column default expression overrides domain's default
-- expression, so no table rewrite
ALTER TABLE test_add_domain_col ADD COLUMN c domain6 DEFAULT 14;
ALTER TABLE test_add_domain_col ADD COLUMN c1 domain8 DEFAULT 13; -- no table rewrite
SELECT attnum, attname, atthasmissing, atthasdef, attmissingval
FROM pg_attribute
WHERE attnum > 0 AND attrelid = 'test_add_domain_col'::regclass AND attisdropped is false
AND atthasmissing
ORDER BY attnum;
attnum | attname | atthasmissing | atthasdef | attmissingval
--------+---------+---------------+-----------+---------------
3 | b | t | t | {12}
4 | c | t | t | {14}
5 | c1 | t | t | {13}
(3 rows)
-- We need to rewrite the table whenever domain default contains volatile expression
ALTER TABLE test_add_domain_col ADD COLUMN d domain6;
NOTICE: rewriting table test_add_domain_col for reason 2
-- We need to rewrite the table whenever domain constraint expression contains volatile expression
ALTER TABLE test_add_domain_col ADD COLUMN e domain7 default 14;
NOTICE: rewriting table test_add_domain_col for reason 2
ALTER TABLE test_add_domain_col ADD COLUMN f domain7;
NOTICE: rewriting table test_add_domain_col for reason 2
-- domain with both volatile and non-volatile CHECK constraints: the
-- volatile one forces a table rewrite
CREATE DOMAIN domain9 AS int CHECK(value > 10) CHECK((value + random(min=>1::int, max=>1)) > 0);
ALTER TABLE test_add_domain_col ADD COLUMN g domain9 DEFAULT 14;
NOTICE: rewriting table test_add_domain_col for reason 2
-- virtual generated columns cannot have domain types
ALTER TABLE test_add_domain_col ADD COLUMN h domain5
GENERATED ALWAYS AS (a + 20) VIRTUAL; -- error
ERROR: virtual generated column "h" cannot have a domain type
DROP TABLE t2;
DROP TABLE test_add_domain_col;
DROP DOMAIN domain1;
DROP DOMAIN domain2;
DROP DOMAIN domain3;
DROP DOMAIN domain4;
DROP DOMAIN domain5;
DROP DOMAIN domain6;
DROP DOMAIN domain7;
DROP DOMAIN domain8;
DROP DOMAIN domain9;
DROP FUNCTION foo(INT);
-- Fall back to full rewrite for volatile expressions
CREATE TABLE T(pk INT NOT NULL PRIMARY KEY);

51
src/test/regress/sql/fast_default.sql
View file

@ -287,11 +287,62 @@ ORDER BY attnum;
SELECT a, b, length(c) = 3 as c_ok, d, e >= 10 as e_ok FROM t2;
-- test fast default over domains with constraints
CREATE DOMAIN domain5 AS int CHECK(value > 10) DEFAULT 8;
CREATE DOMAIN domain6 as int CHECK(value > 10) DEFAULT random(min=>11, max=>100);
CREATE DOMAIN domain7 as int CHECK((value + random(min=>11::int, max=>11)) > 12);
CREATE DOMAIN domain8 as int NOT NULL;
CREATE TABLE test_add_domain_col(a int);
-- succeeds despite constraint-violating default because table is empty
ALTER TABLE test_add_domain_col ADD COLUMN a1 domain5;
ALTER TABLE test_add_domain_col DROP COLUMN a1;
INSERT INTO test_add_domain_col VALUES(1),(2);
-- tests with non-empty table
ALTER TABLE test_add_domain_col ADD COLUMN b domain5; -- table rewrite, then fail
ALTER TABLE test_add_domain_col ADD COLUMN b domain8; -- table rewrite, then fail
ALTER TABLE test_add_domain_col ADD COLUMN b domain5 DEFAULT 1; -- table rewrite, then fail
ALTER TABLE test_add_domain_col ADD COLUMN b domain5 DEFAULT 12; -- ok, no table rewrite
-- explicit column default expression overrides domain's default
-- expression, so no table rewrite
ALTER TABLE test_add_domain_col ADD COLUMN c domain6 DEFAULT 14;
ALTER TABLE test_add_domain_col ADD COLUMN c1 domain8 DEFAULT 13; -- no table rewrite
SELECT attnum, attname, atthasmissing, atthasdef, attmissingval
FROM pg_attribute
WHERE attnum > 0 AND attrelid = 'test_add_domain_col'::regclass AND attisdropped is false
AND atthasmissing
ORDER BY attnum;
-- We need to rewrite the table whenever domain default contains volatile expression
ALTER TABLE test_add_domain_col ADD COLUMN d domain6;
-- We need to rewrite the table whenever domain constraint expression contains volatile expression
ALTER TABLE test_add_domain_col ADD COLUMN e domain7 default 14;
ALTER TABLE test_add_domain_col ADD COLUMN f domain7;
-- domain with both volatile and non-volatile CHECK constraints: the
-- volatile one forces a table rewrite
CREATE DOMAIN domain9 AS int CHECK(value > 10) CHECK((value + random(min=>1::int, max=>1)) > 0);
ALTER TABLE test_add_domain_col ADD COLUMN g domain9 DEFAULT 14;
-- virtual generated columns cannot have domain types
ALTER TABLE test_add_domain_col ADD COLUMN h domain5
GENERATED ALWAYS AS (a + 20) VIRTUAL; -- error
DROP TABLE t2;
DROP TABLE test_add_domain_col;
DROP DOMAIN domain1;
DROP DOMAIN domain2;
DROP DOMAIN domain3;
DROP DOMAIN domain4;
DROP DOMAIN domain5;
DROP DOMAIN domain6;
DROP DOMAIN domain7;
DROP DOMAIN domain8;
DROP DOMAIN domain9;
DROP FUNCTION foo(INT);
-- Fall back to full rewrite for volatile expressions