PostgreSQL数据库TableAM——systable操作

╰半夏微凉° 2022-10-12 04:19 222阅读 0赞

## 举例 ##

以get\_extension\_oid函数为例，该函数通过extension的名字查找其oid。首先调用[table\_open][table_open]函数打开oid为3079的表，后续通过如下步骤来操作系统表：

1.  ScanKeyInit(&entry\[0\],Anum\_pg\_extension\_extname,BTEqualStrategyNumber, F\_NAMEEQ,CStringGetDatum(extname));
2.  scandesc = systable\_beginscan(rel, ExtensionNameIndexId, true,NULL, 1, entry);
3.  tuple = systable\_getnext(scandesc);
4.  systable\_endscan(scandesc);

Oid get_extension_oid(const char *extname, bool missing_ok) { 
    	Oid			result;
    	Relation	rel;
    	SysScanDesc scandesc;
    	HeapTuple	tuple;
    	ScanKeyData entry[1];
    	rel = table_open(ExtensionRelationId, AccessShareLock);
    	ScanKeyInit(&entry[0],Anum_pg_extension_extname,BTEqualStrategyNumber, F_NAMEEQ,CStringGetDatum(extname));
    	scandesc = systable_beginscan(rel, ExtensionNameIndexId, true,NULL, 1, entry);
    	tuple = systable_getnext(scandesc);
    	/* We assume that there can be at most one matching tuple */
    	if (HeapTupleIsValid(tuple))
    		result = ((Form_pg_extension) GETSTRUCT(tuple))->oid;
    	else
    		result = InvalidOid;
    	systable_endscan(scandesc);
    	table_close(rel, AccessShareLock);
    	if (!OidIsValid(result) && !missing_ok)
    		ereport(ERROR,(errcode(ERRCODE_UNDEFINED_OBJECT),errmsg("extension \"%s\" does not exist",extname)));
    	return result;
    }

ScanKeyInit(&entry\[0\],Anum\_pg\_extension\_extname,BTEqualStrategyNumber, F\_NAMEEQ,CStringGetDatum(extname))语句主要用于填充ScanKeyData结构体。  
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scandesc = systable\_beginscan(rel, ExtensionNameIndexId, true, NULL, 1, entry); ExtensionNameIndexId宏定义为3081，是扩展表的索引的oid。主要流程就是打开索引，初始化SysScanDesc结构体并返回。

SysScanDesc systable_beginscan(Relation heapRelation, Oid indexId, bool indexOK, Snapshot snapshot, int nkeys, ScanKey key) { 
    	SysScanDesc sysscan;
    	Relation	irel;
    	if (indexOK && !IgnoreSystemIndexes && !ReindexIsProcessingIndex(indexId))
    		irel = index_open(indexId, AccessShareLock);
    	else
    		irel = NULL;
    	sysscan = (SysScanDesc) palloc(sizeof(SysScanDescData));
    	sysscan->heap_rel = heapRelation;
    	sysscan->irel = irel;
    	sysscan->slot = table_slot_create(heapRelation, NULL);
    	if (snapshot == NULL){ 
    		Oid			relid = RelationGetRelid(heapRelation);
    		snapshot = RegisterSnapshot(GetCatalogSnapshot(relid));
    		sysscan->snapshot = snapshot;
    	} else { 
    		/* Caller is responsible for any snapshot. */
    		sysscan->snapshot = NULL;
    	}
    	if (irel) { 
    		int			i;
    		for (i = 0; i < nkeys; i++) { 
    			int			j;
    			for (j = 0; j < IndexRelationGetNumberOfAttributes(irel); j++) { 
    				if (key[i].sk_attno == irel->rd_index->indkey.values[j]) { 
    					key[i].sk_attno = j + 1;
    					break;
    				}
    			}
    			if (j == IndexRelationGetNumberOfAttributes(irel))
    				elog(ERROR, "column is not in index");
    		}
    		sysscan->iscan = index_beginscan(heapRelation, irel, snapshot, nkeys, 0);
    		index_rescan(sysscan->iscan, key, nkeys, NULL, 0);
    		sysscan->scan = NULL;
    	} else { 
    		sysscan->scan = table_beginscan_strat(heapRelation, snapshot, nkeys, key, true, false);
    		sysscan->iscan = NULL;
    	}
    	return sysscan;
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2FzbWFydGtpbGxlcg_size_16_color_FFFFFF_t_70_pic_center 1]  
systable\_getnext函数在heap或index扫描中获取下一个元组。如果有打开的索引，通过index\_getnext\_slot函数获取slot，然后使用ExecFetchSlotHeapTuple函数获取元组；如果没有索引，通过table\_scan\_getnextslot函数获取slot，然后使用ExecFetchSlotHeapTuple函数获取元组。

HeapTuple systable_getnext(SysScanDesc sysscan) { 
    	HeapTuple	htup = NULL;
    	if (sysscan->irel) { 
    		if (index_getnext_slot(sysscan->iscan, ForwardScanDirection, sysscan->slot)){ 
    			bool		shouldFree;
    			htup = ExecFetchSlotHeapTuple(sysscan->slot, false, &shouldFree);
    			Assert(!shouldFree);
    			if (sysscan->iscan->xs_recheck)
    				elog(ERROR, "system catalog scans with lossy index conditions are not implemented");
    		}
    	} else { 
    		if (table_scan_getnextslot(sysscan->scan, ForwardScanDirection, sysscan->slot)) { 
    			bool		shouldFree;
    			htup = ExecFetchSlotHeapTuple(sysscan->slot, false, &shouldFree);
    			Assert(!shouldFree);
    		}
    	}
    	return htup;
    }

[table_open]: https://feishujun.blog.csdn.net/article/details/118313480
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