PostgreSQL源码解读（88）-查询语句#73（SeqNext函数#1）
本节介绍了SeqNext函数的主要实现逻辑以及该函数中初始化相关数据结构的实现逻辑。SeqNext函数作为参数传递到函数ExecScan中，执行实际的扫描操作。
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一、数据结构
TupleTableSlot
Tuple Table Slot,用于存储元组相关信息
/* base tuple table slot type */
typedef struct TupleTableSlot
{
NodeTag type;//Node标记
#define FIELDNO_TUPLETABLESLOT_FLAGS 1
uint16 tts_flags; /* 布尔状态;Boolean states */
#define FIELDNO_TUPLETABLESLOT_NVALID 2
AttrNumber tts_nvalid; /* 在tts_values中有多少有效的values;# of valid values in tts_values */
const TupleTableSlotOps *const tts_ops; /* 实现一个slot的成本;implementation of slot */
#define FIELDNO_TUPLETABLESLOT_TUPLEDESCRIPTOR 4
TupleDesc tts_tupleDescriptor; /* slot的元组描述符;slot's tuple descriptor */
#define FIELDNO_TUPLETABLESLOT_VALUES 5
Datum *tts_values; /* 当前属性值;current per-attribute values */
#define FIELDNO_TUPLETABLESLOT_ISNULL 6
bool *tts_isnull; /* 当前属性isnull标记;current per-attribute isnull flags */
MemoryContext tts_mcxt; /*内存上下文; slot itself is in this context */
} TupleTableSlot;
typedef struct tupleDesc
{
int natts; /* tuple中的属性数量;number of attributes in the tuple */
Oid tdtypeid; /* tuple类型的组合类型ID;composite type ID for tuple type */
int32 tdtypmod; /* tuple类型的typmode;typmod for tuple type */
int tdrefcount; /* 依赖计数,如为-1,则没有依赖;reference count, or -1 if not counting */
TupleConstr *constr; /* 约束,如无则为NULL;constraints, or NULL if none */
/* attrs[N] is the description of Attribute Number N+1 */
//attrs[N]是第N+1个属性的描述符
FormData_pg_attribute attrs[FLEXIBLE_ARRAY_MEMBER];
} *TupleDesc;
HeapTuple
HeapTupleData是一个指向元组的内存数据结构
HeapTuple是指向HeapTupleData指针
/*
* HeapTupleData is an in-memory data structure that points to a tuple.
* HeapTupleData是一个指向元组的内存数据结构。
*
* There are several ways in which this data structure is used:
* 使用这种数据结构有几种方式:
*
* * Pointer to a tuple in a disk buffer: t_data points directly into the
* buffer (which the code had better be holding a pin on, but this is not
* reflected in HeapTupleData itself).
* 指向磁盘缓冲区中的一个tuple的指针:
* t_data点直接指向缓冲区(代码最好将pin放在缓冲区中，但这在HeapTupleData本身中没有反映出来)。
*
* * Pointer to nothing: t_data is NULL. This is used as a failure indication
* in some functions.
* 没有指针:
* t_data是空的。用于在一些函数中作为故障指示。
*
* * Part of a palloc'd tuple: the HeapTupleData itself and the tuple
* form a single palloc'd chunk. t_data points to the memory location
* immediately following the HeapTupleData struct (at offset HEAPTUPLESIZE).
* This is the output format of heap_form_tuple and related routines.
* palloc'd tuple的一部分:HeapTupleData本身和tuple形成一个单一的palloc'd chunk。
* t_data指向HeapTupleData结构体后面的内存位置(偏移HEAPTUPLESIZE)。
* 这是heap_form_tuple和相关例程的输出格式。
*
* * Separately allocated tuple: t_data points to a palloc'd chunk that
* is not adjacent to the HeapTupleData. (This case is deprecated since
* it's difficult to tell apart from case #1. It should be used only in
* limited contexts where the code knows that case #1 will never apply.)
* 单独分配的tuple:
* t_data指向一个与HeapTupleData不相邻的palloc数据块。
* (这个情况已废弃不用，因为很难与第一种情况中进行区分。
* 它应该只在代码知道第一种情况永远不会适用的有限上下文中使用。
*
* * Separately allocated minimal tuple: t_data points MINIMAL_TUPLE_OFFSET
* bytes before the start of a MinimalTuple. As with the previous case,
* this can't be told apart from case #1 by inspection; code setting up
* or destroying this representation has to know what it's doing.
* 独立分配的最小元组:
* t_data指向MinimalTuple开始前偏移MINIMAL_TUPLE_OFFSET个字节的位置。
* 与前一种情况一样，不能通过检查与第一种情况相区别;
* 设置或销毁这种表示的代码必须知道它在做什么。
*
* t_len should always be valid, except in the pointer-to-nothing case.
* t_self and t_tableOid should be valid if the HeapTupleData points to
* a disk buffer, or if it represents a copy of a tuple on disk. They
* should be explicitly set invalid in manufactured tuples.
* t_len应该总是有效的，除非在指针为NULL。
* 如果HeapTupleData指向磁盘缓冲区，或者它表示磁盘上元组的副本，那么t_self和t_tableOid应该是有效的。
* 它们应该显式地在制造的元组中设置为无效。
*/
typedef struct HeapTupleData
{
uint32 t_len; /* *t_data指针的长度;length of *t_data */
ItemPointerData t_self; /* SelfItemPointer */
Oid t_tableOid; /* 该元组所属的table;table the tuple came from */
#define FIELDNO_HEAPTUPLEDATA_DATA 3
HeapTupleHeader t_data; /* 指向元组的header&数据;-> tuple header and data */
} HeapTupleData;
typedef HeapTupleData *HeapTuple;
#define HEAPTUPLESIZE MAXALIGN(sizeof(HeapTupleData))
HeapScanDesc
HeapScanDesc是指向HeapScanDescData结构体的指针
typedef struct HeapScanDescData
{
/* scan parameters */
Relation rs_rd; /* 堆表描述符;heap relation descriptor */
Snapshot rs_snapshot; /* 快照;snapshot to see */
int rs_nkeys; /* 扫描键数;number of scan keys */
ScanKey rs_key; /* 扫描键数组;array of scan key descriptors */
bool rs_bitmapscan; /* bitmap scan=>T;true if this is really a bitmap scan */
bool rs_samplescan; /* sample scan=>T;true if this is really a sample scan */
bool rs_pageatatime; /* 是否验证可见性(MVCC机制);verify visibility page-at-a-time? */
bool rs_allow_strat; /* 是否允许访问策略的使用;allow or disallow use of access strategy */
bool rs_allow_sync; /* 是否允许syncscan的使用;allow or disallow use of syncscan */
bool rs_temp_snap; /* 是否在扫描结束后取消快照"登记";unregister snapshot at scan end? */
/* state set up at initscan time */
//在initscan时配置的状态
BlockNumber rs_nblocks; /* rel中的blocks总数;total number of blocks in rel */
BlockNumber rs_startblock; /* 开始的block编号;block # to start at */
BlockNumber rs_numblocks; /* 最大的block编号;max number of blocks to scan */
/* rs_numblocks is usually InvalidBlockNumber, meaning "scan whole rel" */
//rs_numblocks通常值为InvalidBlockNumber,意味着扫描整个rel
BufferAccessStrategy rs_strategy; /* 读取时的访问场景;access strategy for reads */
bool rs_syncscan; /* 在syncscan逻辑处理时是否报告位置;report location to syncscan logic? */
/* scan current state */
//扫描时的当前状态
bool rs_inited; /* 如为F,则扫描尚未初始化;false = scan not init'd yet */
HeapTupleData rs_ctup; /* 当前扫描的tuple;current tuple in scan, if any */
BlockNumber rs_cblock; /* 当前扫描的block;current block # in scan, if any */
Buffer rs_cbuf; /* 当前扫描的buffer;current buffer in scan, if any */
/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
//注意:如果rs_cbuf<>InvalidBuffer,在buffer设置pin
ParallelHeapScanDesc rs_parallel; /* 并行扫描信息;parallel scan information */
/* these fields only used in page-at-a-time mode and for bitmap scans */
//下面的变量只用于page-at-a-time模式以及位图扫描
int rs_cindex; /* 在vistuples中的当前元组索引;current tuple's index in vistuples */
int rs_ntuples; /* page中的可见元组计数;number of visible tuples on page */
OffsetNumber rs_vistuples[MaxHeapTuplesPerPage]; /* 元组的偏移;their offsets */
} HeapScanDescData;
/* struct definitions appear in relscan.h */
typedef struct HeapScanDescData *HeapScanDesc;
ScanState
ScanState扩展了对表示底层关系扫描的节点类型的PlanState。
/* ----------------
* ScanState information
*
* ScanState extends PlanState for node types that represent
* scans of an underlying relation. It can also be used for nodes
* that scan the output of an underlying plan node --- in that case,
* only ScanTupleSlot is actually useful, and it refers to the tuple
* retrieved from the subplan.
* ScanState扩展了对表示底层关系扫描的节点类型的PlanState。
* 它还可以用于扫描底层计划节点的输出的节点——在这种情况下，实际上只有ScanTupleSlot有用，它引用从子计划检索到的元组。
*
* currentRelation relation being scanned (NULL if none)
* 正在扫描的relation,如无则为NULL
* currentScanDesc current scan descriptor for scan (NULL if none)
* 当前的扫描描述符,如无则为NULL
* ScanTupleSlot pointer to slot in tuple table holding scan tuple
* 指向tuple table中的slot
* ----------------
*/
typedef struct ScanState
{
PlanState ps; /* its first field is NodeTag */
Relation ss_currentRelation;
HeapScanDesc ss_currentScanDesc;
TupleTableSlot *ss_ScanTupleSlot;
} ScanState;
/* ----------------
* SeqScanState information
* ----------------
*/
typedef struct SeqScanState
{
ScanState ss; /* its first field is NodeTag */
Size pscan_len; /* size of parallel heap scan descriptor */
} SeqScanState;
二、源码解读
SeqNext函数是ExecSeqScan的元组的实际访问方法(ExecScanAccessMtd).这里简单介绍了初始化过程,实际的元组获取过程下节再行介绍.
/* ----------------------------------------------------------------
* SeqNext
*
* This is a workhorse for ExecSeqScan
* 这是ExecSeqScan的实际访问方法(ExecScanAccessMtd)
* ----------------------------------------------------------------
*/
static TupleTableSlot *
SeqNext(SeqScanState *node)
{
HeapTuple tuple;
HeapScanDesc scandesc;
EState *estate;
ScanDirection direction;
TupleTableSlot *slot;
/*
* get information from the estate and scan state
* 从EState和ScanSate中获取相关信息
*/
scandesc = node->ss.ss_currentScanDesc;
estate = node->ss.ps.state;
direction = estate->es_direction;
slot = node->ss.ss_ScanTupleSlot;
if (scandesc == NULL)//如scandesc为NULL,则初始化
{
/*
* We reach here if the scan is not parallel, or if we're serially
* executing a scan that was planned to be parallel.
* 如果扫描不是并行的，或者正在序列化执行计划为并行的扫描，实现逻辑就会到这里。
*/
scandesc = heap_beginscan(node->ss.ss_currentRelation,
estate->es_snapshot,
0, NULL);//扫描前准备,返回HeapScanDesc
node->ss.ss_currentScanDesc = scandesc;//赋值
}
/*
* get the next tuple from the table
* 从数据表中获取下一个tuple
*/
tuple = heap_getnext(scandesc, direction);
/*
* save the tuple and the buffer returned to us by the access methods in
* our scan tuple slot and return the slot. Note: we pass 'false' because
* tuples returned by heap_getnext() are pointers onto disk pages and were
* not created with palloc() and so should not be pfree()'d. Note also
* that ExecStoreHeapTuple will increment the refcount of the buffer; the
* refcount will not be dropped until the tuple table slot is cleared.
* 保存的元组和缓冲区，这些信息通过调用访问方法时返回,同时该方法返回slot。
* 注意:我们传递‘false’，因为heap_getnext()返回的元组是指向磁盘页面的指针，
* 不是用palloc()创建的，所以不应该使用pfree()函数释放。
* 还要注意，ExecStoreHeapTuple将增加缓冲区的refcount;在清除tuple table slot之前不会删除refcount。
*/
if (tuple)//获取了tuple
ExecStoreBufferHeapTuple(tuple, /* 需要存储的tuple;tuple to store */
slot, /* 即将用于存储tuple的slot;slot to store in */
scandesc->rs_cbuf); /* 与该tuple相关联的缓冲区;
buffer associated
* with this tuple */
else
ExecClearTuple(slot);//tuple为NULL,则释放slot
return slot;//返回slot
}
/*
* SeqRecheck -- access method routine to recheck a tuple in EvalPlanQual
* 访问方法在EvalPlanQual中对元组重新检查
*/
static bool
SeqRecheck(SeqScanState *node, TupleTableSlot *slot)
{
/*
* Note that unlike IndexScan, SeqScan never use keys in heap_beginscan
* (and this is very bad) - so, here we do not check are keys ok or not.
* 注意，与IndexScan不同，SeqScan从不使用heap_beginscan中的键(这很糟糕)——因此，这里我们不检查键是否正确。
*/
//直接返回T
return true;
}
/* ----------------
* heap_beginscan - begin relation scan
* heap_beginscan - 开始堆表扫描
*
* heap_beginscan is the "standard" case.
* heap_beginscan是标准情况
*
* heap_beginscan_catalog differs in setting up its own temporary snapshot.
* heap_beginscan_catalog与heap_beginscan不同的是,该方法配置自己的临时快照
*
* heap_beginscan_strat offers an extended API that lets the caller control
* whether a nondefault buffer access strategy can be used, and whether
* syncscan can be chosen (possibly resulting in the scan not starting from
* block zero). Both of these default to true with plain heap_beginscan.
* heap_beginscan_strat提供了一个扩展API，可以让调用者控制是否可以使用非默认的缓冲区访问策略，
* 以及是否可以选择syncscan(可能导致扫描从非0块开始)。
* 对于普通的heap_beginscan，这两个默认值都为T。
*
* heap_beginscan_bm is an alternative entry point for setting up a
* HeapScanDesc for a bitmap heap scan. Although that scan technology is
* really quite unlike a standard seqscan, there is just enough commonality
* to make it worth using the same data structure.
* heap_beginscan_bm是为位图堆扫描设置HeapScanDesc的备选入口点。
* 尽管这种扫描技术与标准的seqscan非常不同，但它有足够的共性，因此值得使用相同的数据结构。
*
* heap_beginscan_sampling is an alternative entry point for setting up a
* HeapScanDesc for a TABLESAMPLE scan. As with bitmap scans, it's worth
* using the same data structure although the behavior is rather different.
* In addition to the options offered by heap_beginscan_strat, this call
* also allows control of whether page-mode visibility checking is used.
* heap_beginscan_sampling是为TABLESAMPLE扫描设置HeapScanDesc的备选入口点。
* 与位图扫描一样，使用相同的数据结构是值得的，尽管其行为相当不同。
* 除了heap_beginscan_strat提供的选项之外，这个调用还允许控制是否使用页面模式可见性检查。
* ----------------
*/
HeapScanDesc
heap_beginscan(Relation relation, Snapshot snapshot,
int nkeys, ScanKey key)
{
return heap_beginscan_internal(relation, snapshot, nkeys, key, NULL,
true, true, true, false, false, false);//标准情况,调用heap_beginscan_internal
}
static HeapScanDesc
heap_beginscan_internal(Relation relation, Snapshot snapshot,//Relation & snapshot
int nkeys, ScanKey key,//键个数&扫描键
ParallelHeapScanDesc parallel_scan,//并行扫描描述符
bool allow_strat,//允许开始?
bool allow_sync,//允许sync扫描?
bool allow_pagemode,//允许页模式?
bool is_bitmapscan,//是否位图扫描
bool is_samplescan,//是否采样扫描
bool temp_snap)//是否使用临时快照
{
HeapScanDesc scan;//堆表扫描描述符
/*
* increment relation ref count while scanning relation
* 在扫描时增加relation依赖计数
*
* This is just to make really sure the relcache entry won't go away while
* the scan has a pointer to it. Caller should be holding the rel open
* anyway, so this is redundant in all normal scenarios...
* 这只是为了确保relcache条目不会在扫描存在指向它的指针时消失。
* 无论如何，调用者都应该保持rel是打开的，所以这在所有正常情况下都是多余的……
*/
RelationIncrementReferenceCount(relation);
/*
* allocate and initialize scan descriptor
* 分配并初始化扫描描述符
*/
scan = (HeapScanDesc) palloc(sizeof(HeapScanDescData));
scan->rs_rd = relation;
scan->rs_snapshot = snapshot;
scan->rs_nkeys = nkeys;
scan->rs_bitmapscan = is_bitmapscan;
scan->rs_samplescan = is_samplescan;
scan->rs_strategy = NULL; /* set in initscan */
scan->rs_allow_strat = allow_strat;
scan->rs_allow_sync = allow_sync;
scan->rs_temp_snap = temp_snap;
scan->rs_parallel = parallel_scan;
/*
* we can use page-at-a-time mode if it's an MVCC-safe snapshot
* 如果快照是MVCC-safte,那么要使用page-at-a-time模式
*/
scan->rs_pageatatime = allow_pagemode && IsMVCCSnapshot(snapshot);
/*
* For a seqscan in a serializable transaction, acquire a predicate lock
* on the entire relation. This is required not only to lock all the
* matching tuples, but also to conflict with new insertions into the
* table. In an indexscan, we take page locks on the index pages covering
* the range specified in the scan qual, but in a heap scan there is
* nothing more fine-grained to lock. A bitmap scan is a different story,
* there we have already scanned the index and locked the index pages
* covering the predicate. But in that case we still have to lock any
* matching heap tuples.
* 对于serializable事务中的seqscan，获取整个关系上的谓词锁。
* 这不仅需要锁定所有匹配的元组，还需要与表中发生的新插入存在冲突。
* 在indexscan中，在覆盖了scan qual中指定的范围的索引页上获取分页锁，但是在堆扫描中没有更细粒度的锁。
* 位图扫描则不同，已经扫描了索引并锁定了覆盖谓词的索引页。但在这种情况下，仍然需要锁定所有匹配的堆元组。
*/
if (!is_bitmapscan)
PredicateLockRelation(relation, snapshot);
/* we only need to set this up once */
//设置relid
scan->rs_ctup.t_tableOid = RelationGetRelid(relation);
/*
* we do this here instead of in initscan() because heap_rescan also calls
* initscan() and we don't want to allocate memory again
* 在这里完成而不是在initscan()中处理是因为heap_rescan也调用initscan()，因此不希望再分配内存
*/
if (nkeys > 0)
scan->rs_key = (ScanKey) palloc(sizeof(ScanKeyData) * nkeys);
else
scan->rs_key = NULL;
//初始化scan
initscan(scan, key, false);
return scan;
}
/* Get the LockTupleMode for a given MultiXactStatus */
#define TUPLOCK_from_mxstatus(status) \
(MultiXactStatusLock[(status)])
/* ----------------------------------------------------------------
* heap support routines
* ----------------------------------------------------------------
*/
/* ----------------
* initscan - scan code common to heap_beginscan and heap_rescan
* initscan - heap_beginscan & heap_rescan的扫描代码
* ----------------
*/
static void
initscan(HeapScanDesc scan, ScanKey key, bool keep_startblock)
{
bool allow_strat;
bool allow_sync;
/*
* Determine the number of blocks we have to scan.
* 确定必须扫描的block数
*
* It is sufficient to do this once at scan start, since any tuples added
* while the scan is in progress will be invisible to my snapshot anyway.
* (That is not true when using a non-MVCC snapshot. However, we couldn't
* guarantee to return tuples added after scan start anyway, since they
* might go into pages we already scanned. To guarantee consistent
* results for a non-MVCC snapshot, the caller must hold some higher-level
* lock that ensures the interesting tuple(s) won't change.)
* 只要在扫描开始时做一次就足够了，因为在扫描进行过程中添加的任何元组对快照都是不可见的。
* (在使用非MVCC快照时不是这样,不能保证返回扫描开始后添加的元组，因为它们可能会存储在已扫描的页面。
* 为了保证非MVCC快照的一致结果，调用者必须持有一些高级锁，以确保有受影响的元组不会改变。)
*/
if (scan->rs_parallel != NULL)
scan->rs_nblocks = scan->rs_parallel->phs_nblocks;
else
scan->rs_nblocks = RelationGetNumberOfBlocks(scan->rs_rd);
/*
* If the table is large relative to NBuffers, use a bulk-read access
* strategy and enable synchronized scanning (see syncscan.c). Although
* the thresholds for these features could be different, we make them the
* same so that there are only two behaviors to tune rather than four.
* (However, some callers need to be able to disable one or both of these
* behaviors, independently of the size of the table; also there is a GUC
* variable that can disable synchronized scanning.)
* 如果表相对于nbuffer较大，则使用批量读取访问策略并启用同步扫描(参见syncscan.c)。
* 尽管这些特性的阈值可能不同，但我们使它们相同，以便只有两种行为可以进行调优，而不是四种。
* (然而，一些调用者需要能够禁用其中一种或两种行为，这与表的大小无关;还有一个GUC变量可以禁用同步扫描。)
*
* Note that heap_parallelscan_initialize has a very similar test; if you
* change this, consider changing that one, too.
* 注意，heap_parallelscan_initialize中有一个非常类似的测试;
* 如果你改变了这个，也应该考虑改变那个。
*/
if (!RelationUsesLocalBuffers(scan->rs_rd) &&
scan->rs_nblocks > NBuffers / 4)
{
allow_strat = scan->rs_allow_strat;
allow_sync = scan->rs_allow_sync;
}
else
allow_strat = allow_sync = false;//设置为F
if (allow_strat)//允许使用访问策略
{
/* During a rescan, keep the previous strategy object. */
//在重新扫描期间,存储先前的策略(strategy)对象
if (scan->rs_strategy == NULL)
scan->rs_strategy = GetAccessStrategy(BAS_BULKREAD);
}
else
{
if (scan->rs_strategy != NULL)
FreeAccessStrategy(scan->rs_strategy);
scan->rs_strategy = NULL;//不允许,则设置为NULL
}
if (scan->rs_parallel != NULL)//使用并行
{
/* For parallel scan, believe whatever ParallelHeapScanDesc says. */
//对于并行扫描,使用ParallelHeapScanDesc中的变量
scan->rs_syncscan = scan->rs_parallel->phs_syncscan;
}
else if (keep_startblock)
{
/*
* When rescanning, we want to keep the previous startblock setting,
* so that rewinding a cursor doesn't generate surprising results.
* Reset the active syncscan setting, though.
* 当重新扫描时，希望保持先前的startblock设置，以便重新回退游标,这样不会产生令人惊讶的结果。
* 不过，注意重置活动syncscan的设置。
*/
scan->rs_syncscan = (allow_sync && synchronize_seqscans);
}
else if (allow_sync && synchronize_seqscans)
{
scan->rs_syncscan = true;
scan->rs_startblock = ss_get_location(scan->rs_rd, scan->rs_nblocks);
}
else
{
scan->rs_syncscan = false;
scan->rs_startblock = 0;
}
scan->rs_numblocks = InvalidBlockNumber;
scan->rs_inited = false;
scan->rs_ctup.t_data = NULL;
ItemPointerSetInvalid(&scan->rs_ctup.t_self);
scan->rs_cbuf = InvalidBuffer;
scan->rs_cblock = InvalidBlockNumber;
/* page-at-a-time fields are always invalid when not rs_inited */
//page-at-a-time相关的域通常设置为无效值
/*
* copy the scan key, if appropriate
* 如需要,拷贝扫描键
*/
if (key != NULL)
memcpy(scan->rs_key, key, scan->rs_nkeys * sizeof(ScanKeyData));
/*
* Currently, we don't have a stats counter for bitmap heap scans (but the
* underlying bitmap index scans will be counted) or sample scans (we only
* update stats for tuple fetches there)
* 目前，没有一个用于位图堆扫描的统计计数器(但是将计算底层的位图索引扫描)
* 或样本扫描(只对那里的元组读取更新统计数据)
*/
if (!scan->rs_bitmapscan && !scan->rs_samplescan)
pgstat_count_heap_scan(scan->rs_rd);
}
三、跟踪分析
测试脚本如下
testdb=# explain select dw.*,grjf.grbh,grjf.xm,grjf.ny,grjf.je
testdb-# from t_dwxx dw,lateral (select gr.grbh,gr.xm,jf.ny,jf.je
testdb(# from t_grxx gr inner join t_jfxx jf
testdb(# on gr.dwbh = dw.dwbh
testdb(# and gr.grbh = jf.grbh) grjf
testdb-# order by dw.dwbh;
QUERY PLAN
------------------------------------------------------------------------------------------
Sort (cost=20070.93..20320.93 rows=100000 width=47)
Sort Key: dw.dwbh
-> Hash Join (cost=3754.00..8689.61 rows=100000 width=47)
Hash Cond: ((gr.dwbh)::text = (dw.dwbh)::text)
-> Hash Join (cost=3465.00..8138.00 rows=100000 width=31)
Hash Cond: ((jf.grbh)::text = (gr.grbh)::text)
-> Seq Scan on t_jfxx jf (cost=0.00..1637.00 rows=100000 width=20)
-> Hash (cost=1726.00..1726.00 rows=100000 width=16)
-> Seq Scan on t_grxx gr (cost=0.00..1726.00 rows=100000 width=16)
-> Hash (cost=164.00..164.00 rows=10000 width=20)
-> Seq Scan on t_dwxx dw (cost=0.00..164.00 rows=10000 width=20)
(11 rows)
启动gdb,设置断点,进入SeqNext
(gdb) b SeqNext
Breakpoint 1 at 0x7156b2: file nodeSeqscan.c, line 60.
(gdb) c
Continuing.
Breakpoint 1, SeqNext (node=0x2ed1588) at nodeSeqscan.c:60
60 scandesc = node->ss.ss_currentScanDesc;
变量赋值
60 scandesc = node->ss.ss_currentScanDesc;
(gdb) n
61 estate = node->ss.ps.state;
(gdb)
62 direction = estate->es_direction;
(gdb)
63 slot = node->ss.ss_ScanTupleSlot;
(gdb)
65 if (scandesc == NULL)
scandesc为NULL,进入初始化,调用heap_beginscan
(gdb) p scandesc
$1 = (HeapScanDesc) 0x0
进入heap_beginscan/heap_beginscan_internal函数
(gdb) n
71 scandesc = heap_beginscan(node->ss.ss_currentRelation,
(gdb) step
heap_beginscan (relation=0x7fb27c488a90, snapshot=0x2e0b8f0, nkeys=0, key=0x0) at heapam.c:1407
1407 return heap_beginscan_internal(relation, snapshot, nkeys, key, NULL,
(gdb) step
heap_beginscan_internal (relation=0x7fb27c488a90, snapshot=0x2e0b8f0, nkeys=0, key=0x0, parallel_scan=0x0,
allow_strat=true, allow_sync=true, allow_pagemode=true, is_bitmapscan=false, is_samplescan=false, temp_snap=false)
at heapam.c:1469
1469 RelationIncrementReferenceCount(relation);
heap_beginscan_internal->增加relation参考计数
1469 RelationIncrementReferenceCount(relation);
(gdb) n
heap_beginscan_internal->初始化HeapScanDesc结构体
1474 scan = (HeapScanDesc) palloc(sizeof(HeapScanDescData));
(gdb)
1476 scan->rs_rd = relation;
(gdb)
1477 scan->rs_snapshot = snapshot;
(gdb)
1478 scan->rs_nkeys = nkeys;
(gdb)
1479 scan->rs_bitmapscan = is_bitmapscan;
(gdb)
1480 scan->rs_samplescan = is_samplescan;
(gdb)
1481 scan->rs_strategy = NULL; /* set in initscan */
(gdb)
1482 scan->rs_allow_strat = allow_strat;
(gdb)
1483 scan->rs_allow_sync = allow_sync;
(gdb)
1484 scan->rs_temp_snap = temp_snap;
(gdb)
1485 scan->rs_parallel = parallel_scan;
(gdb)
1490 scan->rs_pageatatime = allow_pagemode && IsMVCCSnapshot(snapshot);
(gdb)
1503 if (!is_bitmapscan)
heap_beginscan_internal->非位图扫描,谓词锁定
1503 if (!is_bitmapscan)
(gdb) p is_bitmapscan
$2 = false
(gdb) n
1504 PredicateLockRelation(relation, snapshot);
(gdb)
1507 scan->rs_ctup.t_tableOid = RelationGetRelid(relation);
heap_beginscan_internal->进入initscan函数
(gdb) n
1513 if (nkeys > 0)
(gdb)
1516 scan->rs_key = NULL;
(gdb)
1518 initscan(scan, key, false);
(gdb) step
initscan (scan=0x2ee4568, key=0x0, keep_startblock=false) at heapam.c:236
236 if (scan->rs_parallel != NULL)
heap_beginscan_internal->relation的大小相对于buffer并不大(<25%),不使用访问策略(批量读取)&同步扫描
(gdb) n
239 scan->rs_nblocks = RelationGetNumberOfBlocks(scan->rs_rd);
(gdb)
253 if (!RelationUsesLocalBuffers(scan->rs_rd) &&
(gdb)
254 scan->rs_nblocks > NBuffers / 4)
(gdb)
253 if (!RelationUsesLocalBuffers(scan->rs_rd) &&
(gdb)
260 allow_strat = allow_sync = false;
heap_beginscan_internal->设置其他变量
312 if (key != NULL)
(gdb)
320 if (!scan->rs_bitmapscan && !scan->rs_samplescan)
(gdb)
321 pgstat_count_heap_scan(scan->rs_rd);
(gdb)
322 }
(gdb)
heap_beginscan_internal->回到heap_beginscan_internal,完成初始化
(gdb) n
heap_beginscan_internal (relation=0x7fb27c488a90, snapshot=0x2e0b8f0, nkeys=0, key=0x0, parallel_scan=0x0,
allow_strat=true, allow_sync=true, allow_pagemode=true, is_bitmapscan=false, is_samplescan=false, temp_snap=false)
at heapam.c:1520
1520 return scan;
(gdb) p *scan
$4 = {rs_rd = 0x7fb27c488a90, rs_snapshot = 0x2e0b8f0, rs_nkeys = 0, rs_key = 0x0, rs_bitmapscan = false,
rs_samplescan = false, rs_pageatatime = true, rs_allow_strat = true, rs_allow_sync = true, rs_temp_snap = false,
rs_nblocks = 726, rs_startblock = 0, rs_numblocks = 4294967295, rs_strategy = 0x0, rs_syncscan = false,
rs_inited = false, rs_ctup = {t_len = 2139062143, t_self = {ip_blkid = {bi_hi = 65535, bi_lo = 65535}, ip_posid = 0},
t_tableOid = 16742, t_data = 0x0}, rs_cblock = 4294967295, rs_cbuf = 0, rs_parallel = 0x0, rs_cindex = 2139062143,
rs_ntuples = 2139062143, rs_vistuples = {32639 }}
(gdb)
DONE!
四、参考资料
PG Document:Query Planning
网站标题：PostgreSQL源码解读（88）-查询语句#73（SeqNext函数#1）
标题来源：http://pwwzsj.com/article/jcssjo.html
