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sqlite3/ext/misc/explain.c

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/*
** 2018-09-16
**
** The author disclaims copyright to this source code. In place of
** a legal notice, here is a blessing:
**
** May you do good and not evil.
** May you find forgiveness for yourself and forgive others.
** May you share freely, never taking more than you give.
**
*************************************************************************
**
** This file demonstrates an eponymous virtual table that returns the
** EXPLAIN output from an SQL statement.
**
** Usage example:
**
** .load ./explain
** SELECT p2 FROM explain('SELECT * FROM sqlite_schema')
** WHERE opcode='OpenRead';
**
** This module was originally written to help simplify SQLite testing,
** by providing an easier means of verifying certain patterns in the
** generated bytecode.
*/
#if !defined(SQLITEINT_H)
#include "sqlite3ext.h"
#endif
SQLITE_EXTENSION_INIT1
#include <assert.h>
#include <string.h>
#ifndef SQLITE_OMIT_VIRTUALTABLE
/* explain_vtab is a subclass of sqlite3_vtab which will
** serve as the underlying representation of a explain virtual table
*/
typedef struct explain_vtab explain_vtab;
struct explain_vtab {
sqlite3_vtab base; /* Base class - must be first */
sqlite3 *db; /* Database connection for this explain vtab */
};
/* explain_cursor is a subclass of sqlite3_vtab_cursor which will
** serve as the underlying representation of a cursor that scans
** over rows of the result from an EXPLAIN operation.
*/
typedef struct explain_cursor explain_cursor;
struct explain_cursor {
sqlite3_vtab_cursor base; /* Base class - must be first */
sqlite3 *db; /* Database connection for this cursor */
char *zSql; /* Value for the EXPLN_COLUMN_SQL column */
sqlite3_stmt *pExplain; /* Statement being explained */
int rc; /* Result of last sqlite3_step() on pExplain */
};
/*
** The explainConnect() method is invoked to create a new
** explain_vtab that describes the explain virtual table.
**
** Think of this routine as the constructor for explain_vtab objects.
**
** All this routine needs to do is:
**
** (1) Allocate the explain_vtab object and initialize all fields.
**
** (2) Tell SQLite (via the sqlite3_declare_vtab() interface) what the
** result set of queries against explain will look like.
*/
static int explainConnect(
sqlite3 *db,
void *pAux,
int argc, const char *const*argv,
sqlite3_vtab **ppVtab,
char **pzErr
){
explain_vtab *pNew;
int rc;
/* Column numbers */
#define EXPLN_COLUMN_ADDR 0 /* Instruction address */
#define EXPLN_COLUMN_OPCODE 1 /* Opcode */
#define EXPLN_COLUMN_P1 2 /* Operand 1 */
#define EXPLN_COLUMN_P2 3 /* Operand 2 */
#define EXPLN_COLUMN_P3 4 /* Operand 3 */
#define EXPLN_COLUMN_P4 5 /* Operand 4 */
#define EXPLN_COLUMN_P5 6 /* Operand 5 */
#define EXPLN_COLUMN_COMMENT 7 /* Comment */
#define EXPLN_COLUMN_SQL 8 /* SQL that is being explained */
rc = sqlite3_declare_vtab(db,
"CREATE TABLE x(addr,opcode,p1,p2,p3,p4,p5,comment,sql HIDDEN)");
if( rc==SQLITE_OK ){
pNew = sqlite3_malloc( sizeof(*pNew) );
*ppVtab = (sqlite3_vtab*)pNew;
if( pNew==0 ) return SQLITE_NOMEM;
memset(pNew, 0, sizeof(*pNew));
pNew->db = db;
}
return rc;
}
/*
** This method is the destructor for explain_cursor objects.
*/
static int explainDisconnect(sqlite3_vtab *pVtab){
sqlite3_free(pVtab);
return SQLITE_OK;
}
/*
** Constructor for a new explain_cursor object.
*/
static int explainOpen(sqlite3_vtab *p, sqlite3_vtab_cursor **ppCursor){
explain_cursor *pCur;
pCur = sqlite3_malloc( sizeof(*pCur) );
if( pCur==0 ) return SQLITE_NOMEM;
memset(pCur, 0, sizeof(*pCur));
pCur->db = ((explain_vtab*)p)->db;
*ppCursor = &pCur->base;
return SQLITE_OK;
}
/*
** Destructor for a explain_cursor.
*/
static int explainClose(sqlite3_vtab_cursor *cur){
explain_cursor *pCur = (explain_cursor*)cur;
sqlite3_finalize(pCur->pExplain);
sqlite3_free(pCur->zSql);
sqlite3_free(pCur);
return SQLITE_OK;
}
/*
** Advance a explain_cursor to its next row of output.
*/
static int explainNext(sqlite3_vtab_cursor *cur){
explain_cursor *pCur = (explain_cursor*)cur;
pCur->rc = sqlite3_step(pCur->pExplain);
if( pCur->rc!=SQLITE_DONE && pCur->rc!=SQLITE_ROW ) return pCur->rc;
return SQLITE_OK;
}
/*
** Return values of columns for the row at which the explain_cursor
** is currently pointing.
*/
static int explainColumn(
sqlite3_vtab_cursor *cur, /* The cursor */
sqlite3_context *ctx, /* First argument to sqlite3_result_...() */
int i /* Which column to return */
){
explain_cursor *pCur = (explain_cursor*)cur;
if( i==EXPLN_COLUMN_SQL ){
sqlite3_result_text(ctx, pCur->zSql, -1, SQLITE_TRANSIENT);
}else{
sqlite3_result_value(ctx, sqlite3_column_value(pCur->pExplain, i));
}
return SQLITE_OK;
}
/*
** Return the rowid for the current row. In this implementation, the
** rowid is the same as the output value.
*/
static int explainRowid(sqlite3_vtab_cursor *cur, sqlite_int64 *pRowid){
explain_cursor *pCur = (explain_cursor*)cur;
*pRowid = sqlite3_column_int64(pCur->pExplain, 0);
return SQLITE_OK;
}
/*
** Return TRUE if the cursor has been moved off of the last
** row of output.
*/
static int explainEof(sqlite3_vtab_cursor *cur){
explain_cursor *pCur = (explain_cursor*)cur;
return pCur->rc!=SQLITE_ROW;
}
/*
** This method is called to "rewind" the explain_cursor object back
** to the first row of output. This method is always called at least
** once prior to any call to explainColumn() or explainRowid() or
** explainEof().
**
** The argv[0] is the SQL statement that is to be explained.
*/
static int explainFilter(
sqlite3_vtab_cursor *pVtabCursor,
int idxNum, const char *idxStr,
int argc, sqlite3_value **argv
){
explain_cursor *pCur = (explain_cursor *)pVtabCursor;
char *zSql = 0;
int rc;
sqlite3_finalize(pCur->pExplain);
pCur->pExplain = 0;
if( sqlite3_value_type(argv[0])!=SQLITE_TEXT ){
pCur->rc = SQLITE_DONE;
return SQLITE_OK;
}
sqlite3_free(pCur->zSql);
pCur->zSql = sqlite3_mprintf("%s", sqlite3_value_text(argv[0]));
if( pCur->zSql ){
zSql = sqlite3_mprintf("EXPLAIN %s", pCur->zSql);
}
if( zSql==0 ){
rc = SQLITE_NOMEM;
}else{
rc = sqlite3_prepare_v2(pCur->db, zSql, -1, &pCur->pExplain, 0);
sqlite3_free(zSql);
}
if( rc ){
sqlite3_finalize(pCur->pExplain);
pCur->pExplain = 0;
sqlite3_free(pCur->zSql);
pCur->zSql = 0;
}else{
pCur->rc = sqlite3_step(pCur->pExplain);
rc = (pCur->rc==SQLITE_DONE || pCur->rc==SQLITE_ROW) ? SQLITE_OK : pCur->rc;
}
return rc;
}
/*
** SQLite will invoke this method one or more times while planning a query
** that uses the explain virtual table. This routine needs to create
** a query plan for each invocation and compute an estimated cost for that
** plan.
*/
static int explainBestIndex(
sqlite3_vtab *tab,
sqlite3_index_info *pIdxInfo
){
int i; /* Loop counter */
int idx = -1; /* Index of a usable == constraint against SQL */
int unusable = 0; /* True if there are unusable constraints on SQL */
pIdxInfo->estimatedRows = 500;
for(i=0; i<pIdxInfo->nConstraint; i++){
struct sqlite3_index_constraint *p = &pIdxInfo->aConstraint[i];
if( p->iColumn!=EXPLN_COLUMN_SQL ) continue;
if( !p->usable ){
unusable = 1;
}else if( p->op==SQLITE_INDEX_CONSTRAINT_EQ ){
idx = i;
}
}
if( idx>=0 ){
/* There exists a usable == constraint against the SQL column */
pIdxInfo->estimatedCost = 10.0;
pIdxInfo->idxNum = 1;
pIdxInfo->aConstraintUsage[idx].argvIndex = 1;
pIdxInfo->aConstraintUsage[idx].omit = 1;
}else if( unusable ){
/* There are unusable constraints against the SQL column. Do not allow
** this plan to continue forward. */
return SQLITE_CONSTRAINT;
}
return SQLITE_OK;
}
/*
** This following structure defines all the methods for the
** explain virtual table.
*/
static sqlite3_module explainModule = {
0, /* iVersion */
0, /* xCreate */
explainConnect, /* xConnect */
explainBestIndex, /* xBestIndex */
explainDisconnect, /* xDisconnect */
0, /* xDestroy */
explainOpen, /* xOpen - open a cursor */
explainClose, /* xClose - close a cursor */
explainFilter, /* xFilter - configure scan constraints */
explainNext, /* xNext - advance a cursor */
explainEof, /* xEof - check for end of scan */
explainColumn, /* xColumn - read data */
explainRowid, /* xRowid - read data */
0, /* xUpdate */
0, /* xBegin */
0, /* xSync */
0, /* xCommit */
0, /* xRollback */
0, /* xFindMethod */
0, /* xRename */
0, /* xSavepoint */
0, /* xRelease */
0, /* xRollbackTo */
0, /* xShadowName */
};
#endif /* SQLITE_OMIT_VIRTUALTABLE */
int sqlite3ExplainVtabInit(sqlite3 *db){
int rc = SQLITE_OK;
#ifndef SQLITE_OMIT_VIRTUALTABLE
rc = sqlite3_create_module(db, "explain", &explainModule, 0);
#endif
return rc;
}
#ifdef _WIN32
__declspec(dllexport)
#endif
int sqlite3_explain_init(
sqlite3 *db,
char **pzErrMsg,
const sqlite3_api_routines *pApi
){
int rc = SQLITE_OK;
SQLITE_EXTENSION_INIT2(pApi);
#ifndef SQLITE_OMIT_VIRTUALTABLE
rc = sqlite3ExplainVtabInit(db);
#endif
return rc;
}
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