使用案例 3 — SELECT 陳述式中的函式呼叫

在where子句中調用函數可以降低查詢性能，當函數是，VOLATILE並且在調用該函數時不使用select關鍵字：

Select * from tab_name where FieldName = FunctionName(parameters);

如果在呼叫函式時使用select陳述式，則會執行索引掃描：

Select * from tab_name where FieldName = ( select FunctionName(parameters) );

該pnr_number字段在rnr_expiry_date表中有一個索引。比較where子句中的值時，會使用索引。

explain analyze select * from perf_user.rnr_expiry_date where pnr_number= 'EE9F41';

"Index Scan using rnr_expiry_date_idx3 on rnr_expiry_date  (cost=0.29..8.31 rows=1 width=72) (actual time=0.020..0.021 rows=1 loops=1)"
"  Index Cond: ((pnr_number)::text = 'EE9F41'::text)"
"Planning Time: 0.063 ms"
"Execution Time: 0.038 ms"

即使欄位上有索引，當呼叫不含select關鍵字的函數時，仍會執行順序掃描。

explain analyze select * from perf_user.rnr_expiry_date where pnr_number= perf_user.return_data();

"Seq Scan on rnr_expiry_date  (cost=0.00..27084.00 rows=1 width=72) (actual time=0.112..135.917 rows=1 loops=1)"
"  Filter: ((pnr_number)::text = (perf_user.return_data())::text)"
"  Rows Removed by Filter: 99999"
"Planning Time: 0.053 ms"
"Execution Time: 136.803 ms"

使用select關鍵字呼叫函數時，會執行索引掃描。

explain analyze select * from perf_user.rnr_expiry_date where pnr_number= (select perf_user.return_data() );

"Index Scan using rnr_expiry_date_idx3 on rnr_expiry_date  (cost=0.55..8.57 rows=1 width=72) (actual time=0.058..0.061 rows=1 loops=1)"
"  Index Cond: ((pnr_number)::text = ($0)::text)"
"  InitPlan 1 (returns $0)"
"    ->  Result  (cost=0.00..0.26 rows=1 width=32) (actual time=0.021..0.022 rows=1 loops=1)"
"Planning Time: 0.147 ms"
"Execution Time: 0.111 ms"