问题
为什么每次fetch游标能从上一次的位置继续？后面用一个简单用例分析原理。
【速查】
恢复扫描需要知道当前页面、上一次扫描到的偏移位置、当前页面一共有几条：
- 当前页面：HeapScanDesc结构中记录了扫到的页面（scan->rs_cblock）
- 上一次扫描到的偏移位置：scan->rs_cindex，注意rs_cindex是每个页面内的可见元组，从0开始算，每个页面都会从0遍历到scan->rs_ntuples为止。
- 当前页面一共有几条：scan->rs_ntuples记录了当前页面有几个vis元组，在heapgetpage函数中计算。
场景一：open curs1 FOR SELECT ...
drop table tf1;
create table tf1(c1 int, c2 int, c3 varchar(32), c4 varchar(32), c5 int);
insert into tf1 values(1,1000, 'China','Dalian', 23000);
insert into tf1 values(2,4000, 'Janpan', 'Tokio', 45000);
insert into tf1 values(3,1500, 'China', 'Xian', 25000);
insert into tf1 values(4,300, 'China', 'Changsha', 24000);
insert into tf1 values(5,400,'USA','New York', 35000);
insert into tf1 values(6,5000, 'USA', 'Bostom', 15000);
drop procedure tproc1;
CREATE OR REPLACE PROCEDURE tproc1() AS $$
DECLARE
curs1 refcursor;
y tf1%ROWTYPE;
BEGIN
open curs1 FOR SELECT * FROM tf1 WHERE c1 > 3;
fetch curs1 into y;
RAISE NOTICE 'curs1 : %', y.c3;
fetch curs1 into y;
RAISE NOTICE 'curs1 : %', y.c3;
END;
$$ LANGUAGE plpgsql;
call tproc1();
1 OPEN
exec_stmt_open中的执行结构
(gdb) p *stmt
$3 = {
cmd_type = PLPGSQL_STMT_OPEN,
lineno = 6,
stmtid = 1,
curvar = 1,
cursor_options = 256,
argquery = 0x0,
query = 0x1824390,
dynquery = 0x0,
params = 0x0
}
(gdb) p *stmt->query
$5 = {
query = 0x1824698 "SELECT * FROM tf1 WHERE c1 > 3",
parseMode = RAW_PARSE_DEFAULT,
plan = 0x0,
paramnos = 0x0,
func = 0x0,
ns = 0x1824570,
expr_simple_expr = 0x0,
expr_simple_type = 0,
expr_simple_typmod = 0,
expr_simple_mutable = false,
target_param = -1,
expr_rw_param = 0x0,
expr_simple_plansource = 0x0,
expr_simple_plan = 0x0,
expr_simple_plan_lxid = 0,
expr_simple_state = 0x0,
expr_simple_in_use = false,
expr_simple_lxid = 0
}
第一步：exec_prepare_plan
exec_stmt_open
exec_prepare_plan
SPI_prepare_extended
_SPI_prepare_plan
raw_parser
CreateCachedPlan
pg_analyze_and_rewrite_withcb
CompleteCachedPlan
SPI_keepplan
exec_simple_check_plan
结果保存在stmt->query->plan
第二步：SPI_cursor_open_with_paramlist
exec_stmt_open
-- 有参数时会构造ParamListInfo返回
-- 这里没参数，返回NULL
setup_param_list
SPI_cursor_open_with_paramlist
SPI_cursor_open_internal
CreateNewPortal
-- 没ParamListInfo一定走generic plan
GetCachedPlan
PortalDefineQuery
-- 拿快照
CommandCounterIncrement
GetTransactionSnapshot
-- 主要是为了执行InitNode
PortalStart
CreateQueryDesc
ExecutorStart
standard_ExecutorStart
CreateExecutorState
InitPlan
ExecInitRangeTable
ExecInitNode
ExecGetResultType
2 FETCH
第一步：找到portal
curvar = (PLpgSQL_var *) (estate->datums[stmt->curvar]);
curname = TextDatumGetCString(curvar->value);
portal = SPI_cursor_find(curname);
第二步：计算fetch几个？
if (stmt->expr)
how_many = exec_eval_integer(estate, stmt->expr, &isnull);
第三步：FETCH
SPI_scroll_cursor_fetch(portal, FETCH_FORWARD, 1)
_SPI_cursor_operation(.., CreateDestReceiver(DestSPI))
PortalRunFetch(portal, FETCH_FORWARD, 1, dest=<spi_printtupDR>)
MarkPortalActive
DoPortalRunFetch
PortalRunSelect(portal, forward=true, count=1, dest=<spi_printtupDR>)
PushActiveSnapshot
ExecutorRun(queryDesc, direction=ForwardScanDirection, count=1, execute_once=false)
-- 配置接受者，现在是SPI
-- SPI会存到_SPI_current->tuptables中dlist
-- 每个元素是 tuptable，tuptable->vals存放HeapTuple
dest->rStartup
spi_dest_startup
-- 这里入参有一个numberTuples=1表示只执行一条
ExecutePlan
for (;;)
-- 这里只执行一次，那么多次fetch是怎么能继续上次执行的？
ExecProcNode
-- 这里只拿一条，拿到就退
if (numberTuples && numberTuples == current_tuple_count)
break;
PopActiveSnapshot
ExecProcNode展开：执行一次
ExecProcNode
ExecProcNodeFirst
ExecSeqScan
ExecScan
for (;;)
ExecScanFetch
SeqNext
-- 第一次进来创建scandesc
if (scandesc == NULL)
scandesc = table_beginscan(...)
-- 开始扫描
table_scan_getnextslot(scandesc, direction, slot)
heap_getnextslot
heapgettup_pagemode()
heapgettup_pagemode执行第一次：
heapgettup_pagemode执行第N次：
所以为什么每次游标fetch都能继续上次的值：
- HeapScanDesc结构中记录了扫到的页面（scan->rs_cblock）、页面中的位置（scan->rs_cindex），注意rs_cindex是每个页面内的可见元组需要，从0开始算，每个页面都会从0遍历到scan->rs_ntuples为止。
- scan->rs_ntuples记录了当前页面有几个vis元组，在heapgetpage函数中计算。
场景二：open curs1 FOR EXECUTE ...
drop table tf1;
create table tf1(c1 int, c2 int, c3 varchar(32), c4 varchar(32), c5 int);
insert into tf1 values(1,1000, 'China','Dalian', 23000);
insert into tf1 values(2,4000, 'Janpan', 'Tokio', 45000);
insert into tf1 values(3,1500, 'China', 'Xian', 25000);
insert into tf1 values(4,300, 'China', 'Changsha', 24000);
insert into tf1 values(5,400,'USA','New York', 35000);
insert into tf1 values(6,5000, 'USA', 'Bostom', 15000);
CREATE OR REPLACE PROCEDURE tproc1() AS $$
DECLARE
curs1 refcursor;
y tf1%ROWTYPE;
BEGIN
open curs1 FOR EXECUTE 'SELECT * FROM tf1 WHERE c1 > $1' using 3;
fetch curs1 into y;
RAISE NOTICE 'curs1 : %', y.c3;
fetch curs1 into y;
RAISE NOTICE 'curs1 : %', y.c3;
END;
$$ LANGUAGE plpgsql;
call tproc1();
OPEN区别
不在执行exec_prepare_plan直接执行exec_dynquery_with_params：
exec_stmt_open
portal = exec_dynquery_with_params
-- 第一步：把表达式计算出来 "SELECT * FROM tf1 WHERE c1 > $1"
-- 因为有可能使用表达式，比如"select * " || "from " || "tf1"
exec_eval_expr
SPI_cursor_parse_open
_SPI_prepare_plan
SPI_cursor_open_internal
CreateNewPortal
GetCachedPlan
-- 注意这里会把plan删了，portal define的时候是用的copy的，计划没有缓存。
ReleaseCachedPlan(cplan, NULL);
stmt_list = copyObject(stmt_list);
PortalDefineQuery(stmt_list)
PortalStart
FETCH区别
exec_stmt_fetch
SPI_scroll_cursor_fetch
_SPI_cursor_operation
PortalRunFetch
这里的portal没有plan
p *portal
$50 = {name = 0x178b550 "<unnamed portal 10>",
prepStmtName = 0x0,
portalContext = 0x1841b00, resowner = 0x172efe8,
cleanup = 0x6cb0d2 <PortalCleanup>,
createSubid = 1, activeSubid = 1, createLevel = 1,
sourceText = 0x1841c00 "SELECT * FROM tf1 WHERE c1 > $1",
commandTag = CMDTAG_SELECT,
qc = {commandTag = CMDTAG_SELECT, nprocessed = 0},
stmts = 0x1841c30, <<<<<<<<< ------- 拷贝的计划在这里，运行时用这里的计划
cplan = 0x0, <<<<<<<<<<< ------- 注意这里没plan，已经清理了
portalParams = 0x18531b8,
queryEnv = 0x0,
strategy = PORTAL_ONE_SELECT,
cursorOptions = 258, run_once = false, status = PORTAL_READY,
portalPinned = false, autoHeld = false,
queryDesc = 0x1853248, tupDesc = 0x1849288, formats = 0x0,
portalSnapshot = 0x0,
holdStore = 0x0, holdContext = 0x0, holdSnapshot = 0x0,
atStart = true, atEnd = false, portalPos = 0,
creation_time = 766486974937570, visible = true}
继续执行
PortalRunFetch
DoPortalRunFetch
PortalRunSelect
ExecutorRun
for (;;)
ExecProcNode
后续流程相同。
![【洛谷 P8802】[蓝桥杯 2022 国 B] 出差 题解（带权无向图+单源最短路+Dijkstra算法+链式前向星+最小堆）](https://img-blog.csdnimg.cn/img_convert/d0ab4dd521491c3c534fac3b7163e9ec.jpeg)
