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Editor to share with you how PostgreSQL to achieve similar CURRENT_DATE function functions, I believe that most people do not know much about it, so share this article for your reference, I hope you will learn a lot after reading this article, let's go to understand it!

I. realization

The implementation part involves transforming SQLValueFunction (setting the new column zz_sysdate to SQLValueFunction in gram.y), parsing SQLValueFunction and some internal processing of PG.

Convert SQLValueFunction

The SQLValueFunction conversion is in the file src/backend/parser/parse_expr.c, and the corresponding function is transformSQLValueFunction. The new / modified codes are as follows:

Static Node * transformSQLValueFunction (ParseState * pstate, SQLValueFunction * svf) {switch (svf- > op) {. Case SVFOP_ZZ_SYSDATE: svf- > type = DATEOID; break

Parsing SQLValueFunction

Parsing SQLValueFunction in the file src/backend/executor/execExprInterp.c, the corresponding function is ExecEvalSQLValueFunction, and the related code is as follows:

VoidExecEvalSQLValueFunction (ExprState * state, ExprEvalStep * op) {switch (svf- > op) {... Case SVFOP_ZZ_SYSDATE: * op- > resvalue = DateADTGetDatum (GetSQLCurrentDate ()); break

Internal processing

The internal processing of PG is in the file src/backend/utils/adt/ruleutils.c, and the corresponding function is get_rule_expr. The related code is as follows:

Static voidget_rule_expr (Node * node, deparse_context * context, bool showimplicit) {. Case T_SQLValueFunction: {SQLValueFunction * svf = (SQLValueFunction *) node; / * * Note: this code knows that typmod for time, timestamp, and * timestamptz just prints as integer. * / switch (svf- > op) {... Case SVFOP_ZZ_SYSDATE: appendStringInfoString (buf, "ZZ_SYSDATE"); break; II, source code interpretation

Parse_expr.c: handle expressions in parser, processing expressions in parsers

ExecExprInterp.c: analytic evaluation of expressions (steps)

Ruleutils.c: Functions to convert stored expressions/querytrees back to source text, transform and store expression / query tree to source text.

These are all the contents of the article "how to achieve functions similar to CURRENT_DATE functions in PostgreSQL". Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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