定義

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Calcite能夠通過解析Sql為SqlNode,再將SqlNode轉(zhuǎn)化為特定數(shù)據(jù)庫(kù)的方言的形式實(shí)現(xiàn)Sql的統(tǒng)一。

實(shí)現(xiàn)

在Calcite中實(shí)現(xiàn)方言轉(zhuǎn)換的主要類是SqlDialect基類，其具體的變量含義如下：

public class SqlDialect {BUILT_IN_OPERATORS_LIST: 支持的內(nèi)置定義函數(shù)或者運(yùn)算符（例如：abs and..）// 列 表的標(biāo)識(shí)符String identifierQuoteString: 標(biāo)識(shí)符的開始符號(hào)String identifierEndQuoteString: 標(biāo)識(shí)符的結(jié)束符號(hào)String identifierEscapedQuote: （暫時(shí)沒有弄明白這個(gè)在做什么，像是字符串中的轉(zhuǎn)義符？）// 常量的標(biāo)識(shí)符String literalQuoteString: 常量的開始符號(hào)String literalEndQuoteString: 常量的結(jié)束符號(hào)String literalEscapedQuote:（暫時(shí)沒有弄明白這個(gè)在做什么，像是字符串中的轉(zhuǎn)義符？）DatabaseProduct databaseProduct: 所屬的數(shù)據(jù)庫(kù)產(chǎn)品NullCollation nullCollation: 在進(jìn)行排序查詢式，空值的返回順序RelDataTypeSystem dataTypeSystem: 數(shù)據(jù)類型// 和解析相關(guān)Casing unquotedCasing: 大小寫轉(zhuǎn)換Casing quotedCasing: 大小寫轉(zhuǎn)換boolean caseSensitive: 是否大小寫敏感(列名 表明 函數(shù)名等)}// 方法區(qū)（不同的數(shù)據(jù)源根據(jù)細(xì)節(jié)的不同實(shí)現(xiàn)自定義的復(fù)寫方法）allowsAsconfigureParserconfigureParsercontainsNonAsciicreatedefaultNullDirectionemptyContextemulateJoinTypeForCrossJoinemulateNullDirectionemulateNullDirectionWithIsNullgetCalendarPolicygetCastSpecgetConformancegetDatabaseProductgetNullCollationgetProductgetQuotedCasinggetQuotinggetSingleRowTableNamegetTypeSystemgetUnquotedCasinghasImplicitTableAliasidentifierNeedsQuoteisCaseSensitivequoteIdentifierquoteIdentifierquoteIdentifierquoteStringLiteralquoteStringLiteralquoteStringLiteralUnicodequoteTimestampLiteralrequiresAliasForFromItemsrewriteSingleValueExprsupportsAggregateFunctionsupportsAliasedValuessupportsCharSetsupportsDataTypesupportsFunctionsupportsGroupByWithCubesupportsGroupByWithRollupsupportsImplicitTypeCoercionsupportsNestedAggregationssupportsOffsetFetchsupportsWindowFunctionsunparseCallunparseDateTimeLiteralunparseFetchUsingAnsiunparseFetchUsingLimitunparseLimitunparseOffsetunparseOffsetFetchunparseSqlDatetimeArithmeticunparseSqlIntervalLiteralunparseSqlIntervalQualifierunparseTopNunquoteStringLiteral

使用方式Demo

/** Returns SqlNode for type in "cast(column as type)", which might be * different between databases by type name, precision etc. * *

If this method returns null, the cast will be omitted. In the default * implementation, this is the case for the NULL type, and therefore * {@code CAST(NULL AS )} is rendered as {@code NULL}. */ public SqlNode getCastSpec(RelDataType type) 這個(gè)方法就可以根據(jù)具體的數(shù)據(jù)源的數(shù)據(jù)類型進(jìn)行轉(zhuǎn)換，例如： @Override public SqlNode getCastSpec(RelDataType type) { switch (type.getSqlTypeName()) { case VARCHAR: // MySQL doesn't have a VARCHAR type, only CHAR. int vcMaxPrecision = this.getTypeSystem().getMaxPrecision(SqlTypeName.CHAR); int precision = type.getPrecision(); if (vcMaxPrecision > 0 && precision > vcMaxPrecision) { precision = vcMaxPrecision; } return new SqlDataTypeSpec( new SqlBasicTypeNameSpec(SqlTypeName.CHAR, precision, SqlParserPos.ZERO), SqlParserPos.ZERO); } return super.getCastSpec(type); } 就可以經(jīng)Sql中的Cast語(yǔ)句Cast為特定的類型： final String query = "select cast(\"product_id\" as varchar(50)), \"product_id\" " + "from \"product\" "; final String expected = "SELECT CAST(`product_id` AS CHAR(50)), `product_id`\n" + "FROM `foodmart`.`product`";// 解析過的SqlNodesqlNode.toSqlString(CalciteSqlDialect.DEFAULT).getSql();

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新聞名稱：ApacheCalcite實(shí)現(xiàn)方言轉(zhuǎn)換的代碼
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