本节介绍了PostgreSQL含有NOT IN查询语句实现ExecMaterial函数中部分依赖的函数。
一、数据结构
SubPlanState
子计划运行期状态
typedef struct SubPlanState
{
NodeTag type;
SubPlan *subplan;
struct PlanState *planstate;
struct PlanState *parent;
ExprState *testexpr;
List *args;
HeapTuple curTuple;
Datum curArray;
TupleDesc descRight;
ProjectionInfo *projLeft;
ProjectionInfo *projRight;
TupleHashTable hashtable;
TupleHashTable hashnulls;
bool havehashrows;
bool havenullrows;
MemoryContext hashtablecxt;
MemoryContext hashtempcxt;
ExprContext *innerecontext;
AttrNumber *keyColIdx;
Oid *tab_eq_funcoids;
Oid *tab_collations;
FmgrInfo *tab_hash_funcs;
FmgrInfo *tab_eq_funcs;
FmgrInfo *lhs_hash_funcs;
FmgrInfo *cur_eq_funcs;
ExprState *cur_eq_comp;
} SubPlanState;
SubPlan
子查询计划
typedef struct SubPlan
{
Expr xpr;//表达式
//从SubLink中拷贝而来
SubLinkType subLinkType;
//组合操作符,转换为可执行的表达式
Node *testexpr;
List *paramIds;
//Plan tree标识
int plan_id;
//EXPLAIN和debug目的的SubPlan标识
char *plan_name;
//用于确定subplan输出类型的额外信息
Oid firstColType;
int32 firstColTypmod;
Oid firstColCollation;
//执行阶段的相关信息
bool useHashTable;
bool unknownEqFalse;
bool parallel_safe;
//用于给子查询传入和传出参数的信息
//setParam和parParam是整型链表(param IDs)
List *setParam;
List *parParam;
List *args;
//估算执行成本
Cost startup_cost;
Cost per_call_cost;
} SubPlan;
SubLinkType
SubLink类型
typedef enum SubLinkType
{
EXISTS_SUBLINK,
ALL_SUBLINK,
ANY_SUBLINK,
ROWCOMPARE_SUBLINK,
EXPR_SUBLINK,
MULTIEXPR_SUBLINK,
ARRAY_SUBLINK,
CTE_SUBLINK
} SubLinkType;
SubLink
SubLink结构体
typedef struct SubLink
{
Expr xpr;
SubLinkType subLinkType;
int subLinkId;
Node *testexpr;
List *operName;
Node *subselect;
int location;
} SubLink;
MaterialState
Material状态
typedef struct MaterialState
{
ScanState ss;
int eflags;
bool eof_underlying;
Tuplestorestate *tuplestorestate;
} MaterialState;
二、源码解读
ExecMaterial
执行物化操作。
static TupleTableSlot *
ExecMaterial(PlanState *pstate)
{
MaterialState *node = castNode(MaterialState, pstate);//物化节点
EState *estate;//运行期状态
ScanDirection dir;//扫描方向
bool forward;//是否往前扫描
Tuplestorestate *tuplestorestate;//Tuplestorestate结构体指针
bool eof_tuplestore;//是否完成?
TupleTableSlot *slot;//存储元组的slot
CHECK_FOR_INTERRUPTS();
estate = node->ss.ps.state;
dir = estate->es_direction;//方向
forward = ScanDirectionIsForward(dir);//是否往前扫描
tuplestorestate = node->tuplestorestate;
if (tuplestorestate == NULL && node->eflags != 0)
{
tuplestorestate = tuplestore_begin_heap(true, false, work_mem);
tuplestore_set_eflags(tuplestorestate, node->eflags);
if (node->eflags & EXEC_FLAG_MARK)
{
int ptrno PG_USED_FOR_ASSERTS_ONLY;
ptrno = tuplestore_alloc_read_pointer(tuplestorestate,
node->eflags);
Assert(ptrno == 1);
}
node->tuplestorestate = tuplestorestate;
}
eof_tuplestore = (tuplestorestate == NULL) ||
tuplestore_ateof(tuplestorestate);
if (!forward && eof_tuplestore)
{
if (!node->eof_underlying)
{
if (!tuplestore_advance(tuplestorestate, forward))
return NULL;
}
eof_tuplestore = false;
}
slot = node->ss.ps.ps_ResultTupleSlot;
if (!eof_tuplestore)
{
if (tuplestore_gettupleslot(tuplestorestate, forward, false, slot))
return slot;
if (forward)
eof_tuplestore = true;
}
if (eof_tuplestore && !node->eof_underlying)
{
PlanState *outerNode;
TupleTableSlot *outerslot;
outerNode = outerPlanState(node);
outerslot = ExecProcNode(outerNode);
if (TupIsNull(outerslot))
{
node->eof_underlying = true;
return NULL;
}
if (tuplestorestate)
tuplestore_puttupleslot(tuplestorestate, outerslot);
ExecCopySlot(slot, outerslot);
return slot;
}
return ExecClearTuple(slot);
}
tuplestore_begin_heap
初始化tuplestore
Tuplestorestate *
tuplestore_begin_heap(bool randomAccess, bool interXact, int maxKBytes)
{
Tuplestorestate *state;
int eflags;
eflags = randomAccess ?
(EXEC_FLAG_BACKWARD | EXEC_FLAG_REWIND) :
(EXEC_FLAG_REWIND);
state = tuplestore_begin_common(eflags, interXact, maxKBytes);
state->copytup = copytup_heap;
state->writetup = writetup_heap;
state->readtup = readtup_heap;
return state;
}
static Tuplestorestate *
tuplestore_begin_common(int eflags, bool interXact, int maxKBytes)
{
Tuplestorestate *state;
state = (Tuplestorestate *) palloc0(sizeof(Tuplestorestate));
state->status = TSS_INMEM;
state->eflags = eflags;
state->interXact = interXact;
state->truncated = false;
state->allowedMem = maxKBytes * 1024L;
state->availMem = state->allowedMem;
state->myfile = NULL;
state->context = CurrentMemoryContext;
state->resowner = CurrentResourceOwner;
state->memtupdeleted = 0;
state->memtupcount = 0;
state->tuples = 0;
state->memtupsize = Max(16384 / sizeof(void *),
ALLOCSET_SEPARATE_THRESHOLD / sizeof(void *) + 1);
state->growmemtuples = true;
state->memtuples = (void **) palloc(state->memtupsize * sizeof(void *));
USEMEM(state, GetMemoryChunkSpace(state->memtuples));
state->activeptr = 0;
state->readptrcount = 1;
state->readptrsize = 8;
state->readptrs = (TSReadPointer *)
palloc(state->readptrsize * sizeof(TSReadPointer));
state->readptrs[0].eflags = eflags;
state->readptrs[0].eof_reached = false;
state->readptrs[0].current = 0;
return state;
}
tuplestore_advance
从tuplestore前进一行
bool
tuplestore_advance(Tuplestorestate *state, bool forward)
{
void *tuple;
bool should_free;
tuple = tuplestore_gettuple(state, forward, &should_free);
if (tuple)
{
if (should_free)
pfree(tuple);
return true;
}
else
{
return false;
}
}
tuplestore_gettupleslot
获取slot
bool
tuplestore_gettupleslot(Tuplestorestate *state, bool forward,
bool copy, TupleTableSlot *slot)
{
MinimalTuple tuple;
bool should_free;
tuple = (MinimalTuple) tuplestore_gettuple(state, forward, &should_free);
if (tuple)
{
if (copy && !should_free)
{
tuple = heap_copy_minimal_tuple(tuple);
should_free = true;
}
ExecStoreMinimalTuple(tuple, slot, should_free);
return true;
}
else
{
ExecClearTuple(slot);
return false;
}
}
tuplestore_gettuple
返回下一个元组
static void *
tuplestore_gettuple(Tuplestorestate *state, bool forward,
bool *should_free)
{
TSReadPointer *readptr = &state->readptrs[state->activeptr];//读取指针
unsigned int tuplen;
void *tup;
Assert(forward || (readptr->eflags & EXEC_FLAG_BACKWARD));
switch (state->status)
{
case TSS_INMEM://内存中
*should_free = false;
if (forward)
{
if (readptr->eof_reached)
return NULL;
if (readptr->current < state->memtupcount)
{
return state->memtuples[readptr->current++];
}
readptr->eof_reached = true;
return NULL;
}
else
{
if (readptr->eof_reached)
{
readptr->current = state->memtupcount;
readptr->eof_reached = false;
}
else
{
if (readptr->current <= state->memtupdeleted)
{
Assert(!state->truncated);
return NULL;
}
readptr->current--;
}
if (readptr->current <= state->memtupdeleted)
{
Assert(!state->truncated);
return NULL;
}
return state->memtuples[readptr->current - 1];
}
break;
case TSS_WRITEFILE://写文件
//如只需要返回NULL则跳过状态变换
if (readptr->eof_reached && forward)
return NULL;
BufFileTell(state->myfile,
&state->writepos_file, &state->writepos_offset);
if (!readptr->eof_reached)
if (BufFileSeek(state->myfile,
readptr->file, readptr->offset,
SEEK_SET) != 0)
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not seek in tuplestore temporary file: %m")));
state->status = TSS_READFILE;
//进入读文件状态的处理逻辑
case TSS_READFILE:
*should_free = true;
if (forward)
{
//往前读
if ((tuplen = getlen(state, true)) != 0)
{
tup = READTUP(state, tuplen);
return tup;
}
else
{
readptr->eof_reached = true;
return NULL;
}
}
if (BufFileSeek(state->myfile, 0, -(long) sizeof(unsigned int),
SEEK_CUR) != 0)
{
readptr->eof_reached = false;
Assert(!state->truncated);
return NULL;
}
tuplen = getlen(state, false);
if (readptr->eof_reached)
{
readptr->eof_reached = false;
//在返回NULL前返回先前已返回的元组
}
else
{
if (BufFileSeek(state->myfile, 0,
-(long) (tuplen + 2 * sizeof(unsigned int)),
SEEK_CUR) != 0)
{
if (BufFileSeek(state->myfile, 0,
-(long) (tuplen + sizeof(unsigned int)),
SEEK_CUR) != 0)
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not seek in tuplestore temporary file: %m")));
Assert(!state->truncated);
return NULL;
}
tuplen = getlen(state, false);
}
if (BufFileSeek(state->myfile, 0,
-(long) tuplen,
SEEK_CUR) != 0)
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not seek in tuplestore temporary file: %m")));
tup = READTUP(state, tuplen);
return tup;
default:
elog(ERROR, "invalid tuplestore state");
return NULL;
}
}
三、跟踪分析
执行SQL：
[pg12@localhost ~]$ psql -d testdb
Timing is on.
Expanded display is used automatically.
psql (12.0)
Type "help" for help.
[local]:5432 pg12@testdb=#
[local]:5432 pg12@testdb=# select * from tbl;
id | value
----+-------
1 | 2
(1 row)
Time: 2.678 ms
[local]:5432 pg12@testdb=# select count(*) from t_big_null;
count
----------
10000001
(1 row)
Time: 679.972 ms
[local]:5432 pg12@testdb=# analyze tbl;
ANALYZE
Time: 64.442 ms
[local]:5432 pg12@testdb=# analyze t_big_null;
ANALYZE
Time: 434.702 ms
[local]:5432 pg12@testdb=#
[local]:5432 pg12@testdb=# select pg_backend_pid();
pg_backend_pid
----------------
18758
(1 row)
Time: 1.990 ms
[local]:5432 pg12@testdb=# select * from tbl a where a.id not in (select b.id from t_big_null b);
启动gdb跟踪
(gdb) b ExecMaterial
Breakpoint 1 at 0x720edb: file nodeMaterial.c, line 41.
(gdb) c
Continuing.
Breakpoint 1, ExecMaterial (pstate=0x1230128) at nodeMaterial.c:41
41 MaterialState *node = castNode(MaterialState, pstate);
(gdb)
单步调试
(gdb) n
49 CHECK_FOR_INTERRUPTS();
(gdb)
54 estate = node->ss.ps.state;
(gdb)
55 dir = estate->es_direction;
(gdb)
56 forward = ScanDirectionIsForward(dir);
(gdb)
57 tuplestorestate = node->tuplestorestate;
(gdb)
62 if (tuplestorestate == NULL && node->eflags != 0)
(gdb)
64 tuplestorestate = tuplestore_begin_heap(true, false, work_mem);
(gdb)
65 tuplestore_set_eflags(tuplestorestate, node->eflags);
(gdb)
66 if (node->eflags & EXEC_FLAG_MARK)
(gdb)
78 node->tuplestorestate = tuplestorestate;
(gdb)
85 eof_tuplestore = (tuplestorestate == NULL) ||
(gdb)
86 tuplestore_ateof(tuplestorestate);
(gdb)
85 eof_tuplestore = (tuplestorestate == NULL) ||
(gdb)
88 if (!forward && eof_tuplestore)
(gdb) p eof_tuplestore
$1 = false
(gdb)
进入tuplestore_gettupleslot
(gdb) n
107 slot = node->ss.ps.ps_ResultTupleSlot;
(gdb)
108 if (!eof_tuplestore)
(gdb)
110 if (tuplestore_gettupleslot(tuplestorestate, forward, false, slot))
(gdb) step
tuplestore_gettupleslot (state=0x3069c18, forward=true, copy=false, slot=0x30687a8)
at tuplestore.c:1084
1084 tuple = (MinimalTuple) tuplestore_gettuple(state, forward, &should_free);
(gdb)
进入tuplestore_gettuple
(gdb) step
tuplestore_gettuple (state=0x3069c18, forward=true, should_free=0x7ffd18474ff7)
at tuplestore.c:906
906 TSReadPointer *readptr = &state->readptrs[state->activeptr];
(gdb)
tuplestore_gettuple->文件读写指针信息
(gdb) n
910 Assert(forward || (readptr->eflags & EXEC_FLAG_BACKWARD));
(gdb) p *readptr
$2 = {eflags = 2, eof_reached = false, current = 0, file = 2139062143,
offset = 9187201950435737471}
tuplestore_gettuple->当前状态为TSS_INMEM
(gdb) n
912 switch (state->status)
(gdb) p *state
$3 = {status = TSS_INMEM, eflags = 2, backward = false, interXact = false,
truncated = false, availMem = 4177896, allowedMem = 4194304, tuples = 0, myfile = 0x0,
context = 0x3067da0, resowner = 0x2fa62c8, copytup = 0xaba7bd <copytup_heap>,
writetup = 0xaba811 <writetup_heap>, readtup = 0xaba9d9 <readtup_heap>,
memtuples = 0x3051e90, memtupdeleted = 0, memtupcount = 0, memtupsize = 2048,
growmemtuples = true, readptrs = 0x3077f70, activeptr = 0, readptrcount = 1,
readptrsize = 8, writepos_file = 0, writepos_offset = 0}
(gdb) p state->status
$4 = TSS_INMEM
(gdb)
tuplestore_gettuple->返回NULL
(gdb) n
915 *should_free = false;
(gdb) n
916 if (forward)
(gdb)
918 if (readptr->eof_reached)
(gdb)
920 if (readptr->current < state->memtupcount)
(gdb) p readptr->current
$5 = 0
(gdb) p state->memtupcount
$6 = 0
(gdb) n
925 readptr->eof_reached = true;
(gdb)
926 return NULL;
(gdb)
1062 }
(gdb)
tuplestore_gettupleslot->返回false
(gdb) n
tuplestore_gettupleslot (state=0x3069c18, forward=true, copy=false, slot=0x30687a8)
at tuplestore.c:1086
1086 if (tuple)
(gdb)
1098 ExecClearTuple(slot);
(gdb)
1099 return false;
(gdb)
回到ExecMaterial
(gdb) n
1101 }
(gdb)
ExecMaterial (pstate=0x3068158) at nodeMaterial.c:112
112 if (forward)
(gdb)
113 eof_tuplestore = true;
(gdb)
从outerPlan中获取一行（即从t_big_null中获取一行）
(gdb) n
124 if (eof_tuplestore && !node->eof_underlying)
(gdb) p node->eof_underlying
$7 = false
(gdb) n
133 outerNode = outerPlanState(node);
(gdb)
###
#define innerPlanState(node) (((PlanState *)(node))->righttree)
#define outerPlanState(node) (((PlanState *)(node))->lefttree)
###
134 outerslot = ExecProcNode(outerNode);
(gdb) p outerNode
$8 = (PlanState *) 0x3068270
(gdb) p *outerNode
$9 = {type = T_SeqScanState, plan = 0x3037628, state = 0x3067eb8,
ExecProcNode = 0x6f802a <ExecProcNodeFirst>, ExecProcNodeReal = 0x72b904 <ExecSeqScan>,
instrument = 0x0, worker_instrument = 0x0, worker_jit_instrument = 0x0, qual = 0x0,
lefttree = 0x0, righttree = 0x0, initPlan = 0x0, subPlan = 0x0, chgParam = 0x0,
ps_ResultTupleDesc = 0x3068578, ps_ResultTupleSlot = 0x0, ps_ExprContext = 0x3068388,
ps_ProjInfo = 0x0, scandesc = 0x7fab449cae98,
scanops = 0xc3e780 <TTSOpsBufferHeapTuple>, outerops = 0x0, innerops = 0x0,
resultops = 0xc3e780 <TTSOpsBufferHeapTuple>, scanopsfixed = true,
outeropsfixed = false, inneropsfixed = false, resultopsfixed = true, scanopsset = true,
outeropsset = false, inneropsset = false, resultopsset = true}
(gdb) p *outerNode->state
$10 = {type = T_EState, es_direction = ForwardScanDirection, es_snapshot = 0x2f9cd10,
es_crosscheck_snapshot = 0x0, es_range_table = 0x3042130,
es_range_table_array = 0x3068108, es_range_table_size = 2, es_relations = 0x3068130,
es_rowmarks = 0x0, es_plannedstmt = 0x3042438,
es_sourceText = 0x2f74d88 "select * from tbl a where a.id not in (select b.id from t_big_null b);", es_junkFilter = 0x0, es_output_cid = 0, es_result_relations = 0x0,
es_num_result_relations = 0, es_result_relation_info = 0x0,
es_root_result_relations = 0x0, es_num_root_result_relations = 0,
es_partition_directory = 0x0, es_tuple_routing_result_relations = 0x0,
es_trig_target_relations = 0x0, es_param_list_info = 0x0,
es_param_exec_vals = 0x30680d0, es_queryEnv = 0x0, es_query_cxt = 0x3067da0,
es_tupleTable = 0x3068540, es_processed = 0, es_top_eflags = 16, es_instrument = 0,
es_finished = false, es_exprcontexts = 0x3068448, es_subplanstates = 0x3068950,
es_auxmodifytables = 0x0, es_per_tuple_exprcontext = 0x0, es_epq_active = 0x0,
es_use_parallel_mode = false, es_query_dsa = 0x0, es_jit_flags = 25, es_jit = 0x0,
es_jit_worker_instr = 0x0}
(gdb) p ((PlanState *)node)->righttree
$21 = (struct PlanState *) 0x0
(gdb)
回过头来看执行计划，Materialize Node的lefttree是Seq Scan on public.t_big_null b，righttree为NULL。
[local]:5432 pg12@testdb=# explain verbose select * from tbl a where a.id not in (select b.id from t_big_null b);
QUERY PLAN
-------------------------------------------------------------------------------------------
-
Seq Scan on public.tbl a (cost=0.00..129156.33 rows=1 width=8)
Output: a.id, a.value
Filter: (NOT (SubPlan 1))
SubPlan 1
-> Materialize (cost=0.00..233310.68 rows=9999979 width=4)
Output: b.id
-> Seq Scan on public.t_big_null b (cost=0.00..144247.79 rows=9999979 width=4)
Output: b.id
(8 rows)
Time: 7.681 ms
获取outerslot
(gdb) n
135 if (TupIsNull(outerslot))
(gdb) p *outerslot
$16 = {type = T_TupleTableSlot, tts_flags = 16, tts_nvalid = 0,
tts_ops = 0xc3e780 <TTSOpsBufferHeapTuple>, tts_tupleDescriptor = 0x7fab449cae98,
tts_values = 0x30684f0, tts_isnull = 0x30684f8, tts_mcxt = 0x3067da0, tts_tid = {
ip_blkid = {bi_hi = 0, bi_lo = 0}, ip_posid = 1}, tts_tableOid = 49155}
(gdb) p *outerslot->tts_values
$17 = 0
(gdb) p outerslot->tts_values[1]
$18 = 0
(gdb) p outerslot->tts_values[0]
$19 = 0
(gdb) p *outerslot->tts_tupleDescriptor
$20 = {natts = 1, tdtypeid = 49157, tdtypmod = -1, tdrefcount = 2, constr = 0x0,
attrs = 0x7fab449caeb0}
获取outerslot后，put到tuplestore中
(gdb) p *node
$22 = {ss = {ps = {type = T_MaterialState, plan = 0x3040a60, state = 0x3067eb8,
ExecProcNode = 0x720ecf <ExecMaterial>, ExecProcNodeReal = 0x720ecf <ExecMaterial>,
instrument = 0x0, worker_instrument = 0x0, worker_jit_instrument = 0x0, qual = 0x0,
lefttree = 0x3068270, righttree = 0x0, initPlan = 0x0, subPlan = 0x0,
chgParam = 0x0, ps_ResultTupleDesc = 0x3068690, ps_ResultTupleSlot = 0x30687a8,
ps_ExprContext = 0x0, ps_ProjInfo = 0x0, scandesc = 0x3068578,
scanops = 0xc3e720 <TTSOpsMinimalTuple>, outerops = 0x0, innerops = 0x0,
resultops = 0xc3e720 <TTSOpsMinimalTuple>, scanopsfixed = true,
outeropsfixed = false, inneropsfixed = false, resultopsfixed = true,
scanopsset = true, outeropsset = false, inneropsset = false, resultopsset = true},
ss_currentRelation = 0x0, ss_currentScanDesc = 0x0, ss_ScanTupleSlot = 0x3068868},
eflags = 2, eof_underlying = false, tuplestorestate = 0x3069c18}
(gdb) n
146 if (tuplestorestate)
(gdb)
147 tuplestore_puttupleslot(tuplestorestate, outerslot);
(gdb) p outerslot->tts_values[0]
$23 = 0
(gdb) n
149 ExecCopySlot(slot, outerslot);
(gdb) p outerslot->tts_values[0]
$24 = 0
(gdb) n
150 return slot;
(gdb) p outerslot->tts_values[0]
$25 = 0
(gdb) p slot->tts_values[0]
$26 = 0
(gdb) n
157 }
(gdb)
继续“物化”
(gdb) n
ExecProcNodeFirst (node=0x3068158) at execProcnode.c:446
446 }
(gdb) c
Continuing.
Breakpoint 1, ExecMaterial (pstate=0x3068158) at nodeMaterial.c:41
41 MaterialState *node = castNode(MaterialState, pstate);
(gdb) n
49 CHECK_FOR_INTERRUPTS();
(gdb)
54 estate = node->ss.ps.state;
(gdb)
55 dir = estate->es_direction;
(gdb)
56 forward = ScanDirectionIsForward(dir);
(gdb)
57 tuplestorestate = node->tuplestorestate;
(gdb)
62 if (tuplestorestate == NULL && node->eflags != 0)
(gdb)
85 eof_tuplestore = (tuplestorestate == NULL) ||
(gdb)
86 tuplestore_ateof(tuplestorestate);
(gdb)
85 eof_tuplestore = (tuplestorestate == NULL) ||
(gdb)
88 if (!forward && eof_tuplestore)
(gdb)
107 slot = node->ss.ps.ps_ResultTupleSlot;
(gdb)
108 if (!eof_tuplestore)
(gdb)
124 if (eof_tuplestore && !node->eof_underlying)
(gdb)
133 outerNode = outerPlanState(node);
(gdb) p eof_tuplestore
$27 = true
(gdb) n
134 outerslot = ExecProcNode(outerNode);
(gdb)
135 if (TupIsNull(outerslot))
(gdb)
146 if (tuplestorestate)
(gdb)
147 tuplestore_puttupleslot(tuplestorestate, outerslot);
(gdb)
149 ExecCopySlot(slot, outerslot);
(gdb)
150 return slot;
(gdb) p slot->tts_values[0]
$28 = 2
(gdb)
第一次执行时间较久，第二次相对快2个数量级，需要继续研究。
[local]:5432 pg12@testdb=# select * from tbl a where a.id not in (select b.id from t_big_null b);
id | value
----+-------
(0 rows)
Time: 3633462.666 ms (01:00:33.463) --> 包括了debug的时间，实际时间是5s左右
[local]:5432 pg12@testdb=#
[local]:5432 pg12@testdb=# select * from tbl a where a.id not in (select b.id from t_big_null b);
id | value
----+-------
(0 rows)
Time: 6.480 ms --> 第2+次就快很多
[local]:5432 pg12@testdb=#
DONE
四、参考资料
N/A
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