データ型の異なる列での表結合について
初めまして。
質問させていただきます。
下記の表結合でcolumnA1がNUMBER型でcolumnB1がVARCHAR2型の場合に
結合時に暗黙の型変換が行われるのでしょうか?
SELECT columnA2,columnB2 FROM tableA,tableB WHERE columnA1 = columnB1
また、上記SQLをインデックスを用いて結合したいとしたら、columnA1とcolumnB1のデータ型を
揃えるしか方法はないのでしょうか?
ご回答宜しくお願いします。
ご回答ありがとうございます。
tableA (columnA1 NUMBER pk, ...) 20万レコード
tableB (columnB1 VARCHAR2, columnB2, ...) columnB1,columnB2の複合主キー 6万5千レコード
発行SQLは以下のようなものです。
1.SELECT ... FROM tableA, tableB WHERE columnA1=columnB1 AND columnA1 = 11111
2.SELECT ... FROM tableA, tableB WHERE columnA1=columnB1 AND columnB1 = '11111'
上記SQLの実行計画を見ると
1.は駆動表tableA
tableA : Index Unique Scan
tableB : Tabel Full Scan
2.は駆動表tableB
tableB : Index Range Scan
tableA : Index Unique Scan
となり、コストも2.の方が小さかったです。
結合の際に、暗黙の型変換が行われるのとインデックスを使用することとは関連しているわけではない
ということでしょうか?
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