Use text/template for backfill query generation

[andrew-farries]

Generate the backfill query with a text/template instead of the batchStatementBuilder.

Using a text/template instead of the builder with its Sprintf statements make the structure of the query easier to see and modify as part of #583.

Part of #583

[kvch]

Changes look good. Do you intend to adopt text/template package in all SQL generation parts of pgroll?

[andrew-farries]

Changes look good. Do you intend to adopt text/template package in all SQL generation parts of pgroll?

I don't think it's worth doing everywhere, only for generation of longer SQL statements or functions. The only other place where it's currently used is for trigger and trigger function generation:

pgroll/pkg/migrations/templates/function.go

Lines 5 to 32 in c237001

	const Function = `CREATE OR REPLACE FUNCTION {{ .Name | qi }}()
	RETURNS TRIGGER
	LANGUAGE PLPGSQL
	AS $$
	DECLARE
	{{- $schemaName := .SchemaName }}
	{{- $tableName := .TableName }}
	{{ range $name, $col := .Columns }}
	{{- $name | qi }} {{ $schemaName | qi }}.{{ $tableName | qi}}.{{ $col.Name | qi }}%TYPE := NEW.{{ $col.Name | qi }};
	{{ end -}}
	latest_schema text;
	search_path text;
	BEGIN
	SELECT current_setting
	INTO search_path
	FROM current_setting('search_path');
	IF search_path {{- if eq .Direction "up" }} != {{- else }} = {{- end }} {{ .LatestSchema | ql }} {{ if .TestExpr -}} AND {{ .TestExpr }} {{ end -}} THEN
	NEW.{{ .PhysicalColumn | qi }} = {{ .SQL }};
	{{- if .ElseExpr }}
	ELSE
	{{ .ElseExpr }};
	{{- end }}
	END IF;
	RETURN NEW;
	END; $$
	`

The short SQL statements that are generated elsewhere wouldn't benefit from the extra readability that using text/template brings.