Postgresql源码（125）游标恢复执行的原理分析

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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：

1. 当前页面：HeapScanDesc结构中记录了扫到的页面（scan->rs_cblock）
2. 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。
3. 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 $$
DECLAREcurs1 refcursor;  y tf1%ROWTYPE;                     
BEGINopen curs1 FOR SELECT * FROM tf1 WHERE c1 > 3;fetch curs1 into y; RAISE NOTICE 'curs1 : %', y.c3;fetch curs1 into y; RAISE NOTICE 'curs1 : %', y.c3;
END;
$$ 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_openexec_prepare_planSPI_prepare_extended_SPI_prepare_planraw_parserCreateCachedPlanpg_analyze_and_rewrite_withcbCompleteCachedPlanSPI_keepplanexec_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NULLsetup_param_listSPI_cursor_open_with_paramlistSPI_cursor_open_internalCreateNewPortal-- 没ParamListInfo一定走generic planGetCachedPlanPortalDefineQuery-- 拿快照CommandCounterIncrementGetTransactionSnapshot-- 主要是为了执行InitNodePortalStartCreateQueryDescExecutorStartstandard_ExecutorStartCreateExecutorStateInitPlanExecInitRangeTableExecInitNodeExecGetResultType

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>)MarkPortalActiveDoPortalRunFetchPortalRunSelect(portal, forward=true, count=1, dest=<spi_printtupDR>)PushActiveSnapshotExecutorRun(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HeapTupledest->rStartupspi_dest_startup-- 这里入参有一个numberTuples=1表示只执行一条ExecutePlanfor (;;)-- 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

ExecProcNodeExecProcNodeFirstExecSeqScanExecScanfor (;;)ExecScanFetchSeqNext-- 第一次进来创建scandescif (scandesc == NULL)scandesc = table_beginscan(...)-- 开始扫描table_scan_getnextslot(scandesc, direction, slot)heap_getnextslotheapgettup_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：

1. 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。
2. 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 $$
DECLAREcurs1 refcursor;  y tf1%ROWTYPE;                     
BEGINopen curs1 FOR EXECUTE 'SELECT * FROM tf1 WHERE c1 > $1' using 3;fetch curs1 into y; RAISE NOTICE 'curs1 : %', y.c3;fetch curs1 into y; RAISE NOTICE 'curs1 : %', y.c3;
END;
$$ LANGUAGE plpgsql;call tproc1();

OPEN区别

不在执行exec_prepare_plan直接执行exec_dynquery_with_params：

exec_stmt_openportal = 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_exprSPI_cursor_parse_open_SPI_prepare_planSPI_cursor_open_internalCreateNewPortalGetCachedPlan-- 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_fetchSPI_scroll_cursor_fetch_SPI_cursor_operationPortalRunFetch

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

PortalRunFetchDoPortalRunFetchPortalRunSelectExecutorRunfor (;;)ExecProcNode

后续流程相同。

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