This document describes how spannertest is implemented. It should be considered a companion to the source code.
The purpose of spannertest is to support testing code that uses Cloud Spanner as a client. It aims to be correct and comprehensive (see README.md for status), but does not attempt to be performant. Clarity and simplicity of implementation is a very important property; one should be able to read this document and the code and have a good understanding of how an SQL database may operate in principle.
There are five sections to spannertest:
inmem.go); this implements the same gRPC interface as Cloud Spanner. It handles transitions between the gRPC protobuf types and the types used by the rest of the package.db.go); this has the primitive types such as database, table and row, implements schema management, all writes (insert/update/delete/etc. including DML), and basic reads (based on keys and key ranges).db_query.go); this evaluates a spansql.Query on a database.db_eval.go); this evaluates a spansql.Expr in a specific context (e.g. on a table row).funcs.go).inmem.go)TODO
db.go)A database contains a set of named tables. It also contains a set of named indexes, though spannertest does not do anything with them.
A table contains its own schema (a list of colInfo values). The primary key columns appear first in that list for some implementation conveninence; otherwise the order is as they appear in the CREATE TABLE DDL.
A table also has its data, stored as a list of row values, each of which is a list of data values. The rows are stored in primary key order; this aids in search operations.
A row is a list of raw data values, represented as []interface{}. db.go explains the mapping between Spanner types and Go types. These values are used throughout the other parts of the spannertest implementation, particularly in the expression evaluator.
db_query.go)The query evaluator works by transforming a spansql.Query into a pipeline of transformers (the rowIter interface). This pipeline implements the SQL semantics. For instance, SELECT * FROM X WHERE A > 2 would produce a pipeline that reads rows from X (tableIter), filters them (whereIter), and outputs the full set of columns (selIter). See (*database).Query and (*database.evalSelect).
db_eval.go)The expression evaluator walks a spansql.Expr in a particular “evaluation context” to produce an output value. See evalContext.evalExpr for the main entry point.
This also includes the value comparator (compareVals), which is used for evaluating expressions, for ordering results (ORDER BY), some filtering (SELECT DISTINCT)